Blood Red Tetra

Quick Summary

The Blood Red Tetra is a stunning blackwater specialist that rewards experienced aquarists who can provide consistently soft, acidic water – it is not a good choice for beginners or those with hard tap water, but in the right setup it is one of the most vividly coloured small characins available.

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In this guide:

• Introduction
• Quick Stats
• Appearance
• Natural Habitat
• Tank Size and Setup
• Water Parameters
• Diet and Feeding
• Behaviour and Temperament
• Tank Mates
• Breeding

Introduction

Brittanichthys axelrodi, commonly known as the Blood Red Tetra, is a small characin from the tannin-stained blackwater tributaries of the Rio Negro basin in Brazil. Named after the renowned ichthyologist and publisher Herbert R. Axelrod, this species has long fascinated aquarists with its intense, deep crimson body colouration – a trait that sets it apart from many of its tetra relatives. Despite its modest size, the Blood Red Tetra is far from a generic community fish. It has very specific water chemistry requirements that place it firmly in the specialist category, and keeping it successfully requires a genuine understanding of blackwater aquarium management.

Unlike more forgiving tetras such as the Neon Tetra or the Cardinal Tetra, Brittanichthys axelrodi does not adapt well to moderate or hard water. It thrives only in extremely soft, acidic conditions that replicate the dark, mineral-poor waters of its native habitat. For aquarists who are willing to invest in RO water, botanical preparations, and careful monitoring, the Blood Red Tetra is an exceptionally rewarding species. A well-maintained shoal glowing against a backdrop of leaf litter and driftwood is one of the most striking sights in the freshwater hobby.

This guide covers everything you need to know to keep Blood Red Tetras healthy and thriving, from tank setup and water parameters to diet, breeding, and compatible tank mates. If you are considering this species, read on to understand its particular needs before committing.

Appearance

The Blood Red Tetra is a small, slender-bodied characin that typically reaches between 3 and 3.5 centimetres in total length. Its most striking feature is its colouration: when kept in optimal blackwater conditions, the entire body takes on a deep, rich blood-red hue that is truly remarkable for such a small fish. This colouration is not merely a surface phenomenon – the pigment appears to suffuse through the body tissue, giving the fish an almost translucent, glowing quality when viewed under subdued lighting against a dark background.

The body shape is elongated and laterally compressed, typical of many small characins. The fins are relatively modest in size and largely transparent or lightly tinted with red. Males tend to display slightly more intense colouration and may be marginally slimmer than females, though sexual dimorphism in this species is subtle. Females that are carrying eggs may appear slightly rounder in the belly region when viewed from above.

It is worth noting that colouration in Blood Red Tetras is heavily dependent on water conditions. Fish kept in water that is too hard, too alkaline, or lacking in tannins will appear washed out and pale – a far cry from the spectacular red they display in appropriate blackwater setups. Poor diet, stress, and overly bright lighting will also diminish their colour. This is one of those species where the beauty of the fish is a direct reflection of the quality of the husbandry provided.

Under the right conditions, the contrast between the deep red body and the dark, tea-coloured water of a blackwater aquarium is breathtaking. A shoal of well-conditioned Blood Red Tetras moving through dimly lit water over a bed of fallen leaves is one of the most naturalistic and visually arresting scenes available to the freshwater aquarist.

Natural Habitat

Brittanichthys axelrodi is native to the Rio Negro and its tributaries in the Brazilian state of Amazonas. The Rio Negro is one of the largest blackwater river systems in the world, and its waters are famously dark, acidic, and extremely mineral-poor. The dark colour comes from dissolved humic and tannic acids released by decomposing leaf litter, wood, and other organic matter in the surrounding rainforest. This results in water with a characteristic tea or cola colouration, a pH that can drop below 4.0 in some locations, and negligible levels of dissolved minerals.

In the wild, Blood Red Tetras inhabit the slow-moving, shaded tributaries and flooded forest margins (igapo) of the Rio Negro system. The substrate in these habitats is typically composed of fine white sand covered by a thick layer of decomposing leaves, seed pods, and fallen branches. Aquatic plant growth is often sparse due to the extremely low light levels and acidic conditions, though some species of aquatic mosses and certain epiphytes may be present on submerged wood.

These habitats are characterised by very low biological productivity. The water is extremely soft – often with a general hardness of less than 1 dGH – and conductivity readings are frequently below 10 microsiemens. Despite this apparent barrenness, blackwater habitats support a remarkable diversity of small fish species, many of which have evolved specific adaptations to thrive in these nutrient-poor, acidic conditions. Blood Red Tetras are typically found in loose shoals among submerged root systems and leaf litter, where they feed on tiny invertebrates, microorganisms, and organic detritus.

Understanding this natural habitat is essential for keeping Brittanichthys axelrodi successfully in captivity. This is not a species that will adapt to moderate water conditions – it genuinely requires soft, acidic water to remain healthy and display its full colouration. Aquarists who attempt to keep it in standard community tank conditions with a pH above 7 and moderate hardness will almost invariably see poor colouration, increased susceptibility to disease, and shortened lifespans.

Tank Size and Setup

A minimum tank size of 60 litres is recommended for a small shoal of Blood Red Tetras, though a larger aquarium of 80 to 120 litres will provide more stable water conditions and a more visually impressive display. Stability is particularly important with this species, as the very soft, acidic water they require can be prone to pH fluctuations in small volumes. A longer tank is preferable to a tall one, as these fish primarily occupy the mid-water column and benefit from horizontal swimming space.

The ideal setup for Blood Red Tetras replicates their natural blackwater habitat. Begin with a substrate of fine, light-coloured sand – white or pale sand works beautifully, as it contrasts with the dark water and mirrors the sandy bottoms of Rio Negro tributaries. Avoid gravel that could buffer the water or raise the pH, and avoid any calcareous materials such as coral sand, limestone, or shells, as these will increase hardness and alkalinity.

Driftwood is an essential component of the setup. Use a generous quantity of branches, roots, and tangles to create a complex structure with shaded areas and hiding spots. Pieces of Malaysian driftwood, mopani wood, or spider wood are all suitable. The wood will also contribute tannins to the water, helping to lower the pH and create the desired tea-coloured tint. In addition to driftwood, add a thick layer of dried leaf litter – Indian almond leaves (Terminalia catappa), oak leaves, or beech leaves are all excellent choices. The leaves will decompose slowly, releasing further tannins and providing surfaces for biofilm growth, which the tetras will graze on between feedings.

Alder cones, seed pods (such as those from Cariniana or Sterculia species), and other botanicals can be added to enhance the natural look and further condition the water. These items break down over time and will need periodic replacement.

Lighting should be subdued. Bright lighting will wash out the colour of the fish and does not replicate their natural habitat. Use dim or diffused lighting, or allow floating plants to shade the surface. If you wish to include live plants, choose species that tolerate low light and acidic conditions, such as Java Fern (Microsorum pteropus), Java Moss (Taxiphyllum barbieri), or Bucephalandra species attached to the hardscape. Stem plants requiring high light and CO2 injection are generally not appropriate for this type of setup.

A tight-fitting lid or cover glass is advisable, as small tetras can occasionally jump, particularly when startled. Ensure that the tank is placed in a relatively calm area of the home, away from heavy foot traffic and sudden vibrations, as Blood Red Tetras can be nervous fish that respond poorly to disturbance.

Water Parameters

Water chemistry is the single most critical factor in keeping Blood Red Tetras successfully. This species has extremely narrow tolerances compared to most aquarium fish, and maintaining the correct parameters is non-negotiable for long-term health and colour development.

For most aquarists, achieving these parameters will require the use of reverse osmosis (RO) water or very soft rainwater. Tap water in most regions of the United Kingdom and Europe is far too hard and alkaline for this species. If using RO water, you may need to remineralise it very slightly with a product designed for soft-water species, or simply use it neat, depending on your source water and the buffering capacity of your botanicals. A TDS (total dissolved solids) reading of 10 to 50 ppm is ideal.

The pH can be lowered and maintained naturally through the use of driftwood, leaf litter, and peat filtration. Some aquarists also use commercially available blackwater extracts or tannin supplements. It is important to note that at very low KH values the water has almost no buffering capacity, meaning that pH can swing rapidly if conditions change. This is why a larger tank volume is advantageous – it provides a greater buffer against sudden shifts. Regular, small water changes using conditioned RO water are preferable to infrequent large changes.

Nitrate should be kept as low as possible. In the wild, these fish inhabit waters with virtually undetectable levels of dissolved nitrogen compounds. While zero nitrate is unrealistic in a closed aquarium system, aim to keep it below 10 ppm through regular water changes and careful feeding. Avoid overstocking the aquarium, as the bioload in a blackwater tank should be kept deliberately low.

Temperature stability is also important. Aim for 25 to 27 C as a comfortable range. Avoid rapid temperature fluctuations, which can stress the fish and suppress their immune system. Use a reliable heater with an accurate thermostat, and consider using a digital thermometer for precise monitoring.

Filtration and Equipment

Filtration for a Blood Red Tetra tank should be gentle and efficient. These fish come from slow-moving waters and do not appreciate strong currents. A sponge filter is an excellent choice for smaller setups, as it provides effective biological filtration with minimal flow and also offers surfaces for biofilm growth. For larger tanks, a small hang-on-back filter or a compact canister filter set to a low flow rate will work well. If using a canister or power filter, consider adding a spray bar to diffuse the output and reduce current strength.

Peat filtration is a traditional and effective method for conditioning water in blackwater setups. Aquarium-grade peat can be placed in a filter bag within the filter itself, where it will gradually release humic and tannic acids, softening the water and lowering the pH. If you use this method, monitor the pH regularly, as the rate of acid release can vary depending on the type and quantity of peat used.

A reliable heater is essential. Choose a heater appropriately sized for your tank volume and invest in one with an accurate thermostat. In very soft, acidic water, some lower-quality heaters may be prone to corrosion over time, so opt for a well-regarded brand with glass or titanium construction.

Lighting should be kept low, as discussed in the setup section. Standard aquarium LED fixtures can be dimmed or placed on a timer to limit the photoperiod to around 8 hours per day. Floating plants such as Amazon Frogbit (Limnobium laevigatum) or Red Root Floaters (Phyllanthus fluitans) can help to diffuse light reaching the lower levels of the tank.

An air pump is not strictly necessary if you are using a sponge filter, as the filter itself will provide adequate surface agitation for gas exchange. However, in warmer temperatures where dissolved oxygen levels may drop, a gentle airstone can be beneficial. Avoid vigorous aeration, as it can drive off CO2 and raise the pH in unbuffered water.

Diet and Feeding

In the wild, Blood Red Tetras feed on tiny aquatic invertebrates, insect larvae, microorganisms, and organic detritus found among leaf litter and submerged wood. In captivity, they accept a range of foods, but their small mouth size and somewhat cautious feeding behaviour should be taken into account when selecting a diet.

A high-quality micro-pellet or finely crushed flake food can form the staple of their diet. Look for products with a high proportion of natural ingredients and colour-enhancing compounds such as astaxanthin and spirulina, which can help to maintain and intensify the red colouration. However, dry foods alone are not sufficient – live and frozen foods should be offered regularly to ensure optimal health and condition.

Excellent supplementary foods include baby brine shrimp (Artemia nauplii), microworms, Grindal worms, daphnia, cyclops, and finely chopped frozen bloodworm. Live foods are particularly valuable for conditioning fish for breeding and for encouraging natural feeding behaviour. The biofilm and microfauna that develop on decomposing leaves and botanicals in a well-established blackwater tank also provide an important supplementary food source that the fish will graze on throughout the day.

Feed small amounts two to three times daily rather than a single large feed. Blood Red Tetras have small stomachs and benefit from frequent, modest portions. Remove any uneaten food promptly to prevent water quality degradation, which is especially critical in the soft, low-buffered water these fish require. Overfeeding is a common cause of water quality problems in blackwater setups, so err on the side of caution.

Behaviour and Temperament

Blood Red Tetras are peaceful, somewhat shy shoaling fish. They are not aggressive towards other species or towards each other, though males may engage in mild displays of dominance within the group, flaring their colours and posturing near rivals. These displays are harmless and are, in fact, a sign of healthy, confident fish.

This species must be kept in groups of at least 8 to 10 individuals, and larger shoals of 12 or more are strongly recommended. In smaller numbers, Blood Red Tetras become visibly nervous, spending much of their time hiding and rarely displaying their full colouration. A large group provides the social security these fish need to feel comfortable, and a well-settled shoal will spend its time swimming actively in the mid-water column, exploring leaf litter, and foraging on biofilm.

Blood Red Tetras are not fast swimmers and tend to move through the water in a gentle, almost drifting manner. They prefer dim, shaded areas and will gravitate towards overhanging plants, driftwood tangles, and other areas of cover. In brightly lit tanks with minimal cover, they will be perpetually stressed and prone to hiding behind equipment or in corners.

They are generally compatible with other small, peaceful species that share their preference for soft, acidic water. However, they can be outcompeted for food by more assertive feeders, so choose tank mates carefully to ensure the Blood Red Tetras are getting their share at feeding time. Avoid keeping them with boisterous or fast-moving species that might intimidate them.

Tank Mates

Choosing tank mates for Blood Red Tetras is primarily about matching water chemistry requirements. Any species kept alongside Brittanichthys axelrodi must be comfortable in extremely soft, acidic blackwater conditions – this immediately rules out the vast majority of commonly available aquarium fish. Focus on species from the same or similar Amazonian blackwater habitats, and prioritise small, peaceful species that will not outcompete the Blood Red Tetras for food or territory.

Good tank mates

• Cardinal Tetra – A fellow Rio Negro native that thrives in soft, acidic water and makes an excellent visual complement in a blackwater community.
• Green Neon Tetra – Another small, peaceful blackwater characin from the Rio Negro system with very similar water chemistry requirements.
• Chocolate Gourami – A soft-water specialist that occupies a different area of the tank and shares a preference for dim lighting and tannin-rich water.
• Nannostomus marginatus (Dwarf Pencilfish) – A tiny, gentle pencilfish from similar blackwater habitats that coexists peacefully with small tetras.
• Otocinclus – Small, peaceful algae-eating catfish that do well in soft water and will not bother the tetras.
• Pygmy Corydoras – A diminutive bottom-dweller that tolerates soft, acidic conditions and adds activity to the lower levels of the tank.
• Apistogramma elizabethae – A blackwater-adapted dwarf cichlid that is generally peaceful towards small tetras in a suitably sized tank.
• Marbled Hatchetfish – An upper-water-column species from similar habitats that occupies a different zone and does not compete directly with the tetras.

Fish to avoid

• African Cichlids – Require hard, alkaline water that is completely incompatible with the conditions Blood Red Tetras need.
• Guppies – Prefer harder, more alkaline water and will not thrive in the extremely soft, acidic conditions this species requires.
• Tiger Barbs – Active, boisterous fish that would intimidate and stress Blood Red Tetras and prefer different water conditions.
• Oscars – Far too large and predatory to house with any small tetra species.
• Angelfish – Although Amazonian in origin, adult angelfish are large enough to eat Blood Red Tetras and are too assertive as tank mates.
• Platies – Livebearers that require moderately hard, alkaline water and are entirely unsuitable for blackwater conditions.
• Betta splendens – While some bettas tolerate soft water, their territorial nature and differing care requirements make them a poor match.

Breeding

Breeding Blood Red Tetras in captivity is challenging and is considered an advanced undertaking. Successful breeding requires meticulous attention to water chemistry, careful conditioning of the adults, and a dedicated breeding setup separate from the main display tank.

Like many small characins, Blood Red Tetras are egg-scattering spawners. They do not exhibit parental care and will readily consume their own eggs if given the opportunity. To breed them, set up a small, dimly lit breeding tank of approximately 20 to 30 litres with extremely soft, acidic water. The pH should be in the range of 4.0 to 5.0, with a GH close to 0 and a temperature of 26 to 28 C. Use pure RO water conditioned with Indian almond leaves or peat extract to achieve the desired chemistry. The water should be distinctly tea-coloured.

The breeding tank should contain clumps of fine-leaved plants or spawning mops to receive the eggs, and a mesh or grate at the bottom to prevent the adults from reaching fallen eggs. Filtration should be limited to a small, air-driven sponge filter to avoid disturbing the delicate eggs and fry.

Condition a group of healthy, well-fed adults on a varied diet of live and frozen foods for two to three weeks before introducing a selected pair or small group to the breeding tank in the evening. Spawning, if it occurs, typically takes place in the early morning hours. The female will scatter small, adhesive eggs among the plants or mops, and the male will fertilise them as they are released. After spawning, remove the adults immediately to prevent egg predation.

The eggs are very small and somewhat light-sensitive. Keep the breeding tank in near darkness during the incubation period, which lasts approximately 24 to 36 hours at 27 C. The newly hatched fry are tiny and will initially feed on their yolk sac for a further two to three days before becoming free-swimming. First foods should be infusoria, paramecium, or commercially available liquid fry food, followed by freshly hatched baby brine shrimp once the fry are large enough to accept them.

Growth is slow, and the fry are delicate, requiring pristine water quality and very small, frequent feedings. Partial water changes should be performed carefully using water matched exactly to the parameters of the breeding tank. Even with optimal care, survival rates can be modest, and raising a batch of fry to adulthood is a genuine achievement in the hobby.

Common Diseases

Blood Red Tetras are not especially disease-prone when kept in appropriate conditions, but they are significantly less resilient than hardier tetra species when water quality or chemistry is suboptimal. The vast majority of health problems in this species can be traced back to inappropriate water parameters – particularly water that is too hard, too alkaline, or contaminated with elevated levels of nitrogen compounds.

Ich (white spot disease), caused by the protozoan parasite Ichthyophthirius multifiliis, is one of the most common diseases affecting freshwater fish, and Blood Red Tetras are susceptible if stressed by poor conditions or sudden temperature changes. Raising the temperature gradually to 30 C and using a half-dose of a malachite green or formalin-based treatment is the standard approach, though care must be taken with medication in very soft, acidic water, as some treatments can be more potent – and potentially toxic – at low pH values. Always read product instructions carefully and consider using half the recommended dose initially.

Fungal infections can occur on the body or fins, often as a secondary infection following physical damage or stress. These typically present as white, cotton-like growths. Improving water quality and treating with an antifungal medication is usually effective, but again, dose cautiously in blackwater conditions.

Bacterial infections, including fin rot, can develop in tanks with poor water quality. Symptoms include frayed or disintegrating fins, lethargy, and loss of appetite. Address the underlying water quality issue first, and treat with an appropriate antibacterial product if symptoms persist.

Internal parasites may be present in wild-caught specimens. Signs include weight loss despite normal feeding, white or stringy faeces, and a gradual decline in condition. Antiparasitic medications containing praziquantel or levamisole can be used, but quarantine new arrivals for at least two to four weeks before introducing them to an established tank.

Prevention is always preferable to treatment with this species. Maintain impeccable water quality, avoid sudden changes in temperature or chemistry, quarantine all new fish, and provide a varied, high-quality diet. A well-maintained blackwater tank with stable parameters will go a long way towards keeping Blood Red Tetras healthy throughout their lifespan.

FAQs

Can I keep Blood Red Tetras in a standard community tank?

Not really. Blood Red Tetras require extremely soft, acidic water with a pH typically below 6.0 and a GH close to zero. Most standard community tanks are maintained with moderate hardness and a neutral to slightly alkaline pH, which is unsuitable for this species. If your tank is set up specifically as a blackwater biotope with the appropriate water chemistry, they can be kept alongside other compatible soft-water species, but they should not be added to a typical mixed community aquarium.

Why are my Blood Red Tetras pale and washed out?

The most common reason is inappropriate water chemistry. Blood Red Tetras only display their full, vivid red colouration in very soft, acidic, tannin-stained water. If your water is too hard, too alkaline, or lacks tannins, the fish will appear pale and unimpressive. Other contributing factors include overly bright lighting, stress from inadequate group size, poor diet, and illness. Address the water chemistry first, ensure the tank has plenty of driftwood and leaf litter, dim the lighting, and maintain a shoal of at least 8 to 10 individuals.

How many Blood Red Tetras should I keep together?

A minimum group of 8 to 10 is recommended, but a shoal of 12 or more is ideal. Blood Red Tetras are social fish that rely on the security of the group to feel confident enough to swim openly and display their best colouration. In smaller groups, they tend to be shy, nervous, and prone to hiding. A larger shoal also produces more natural and visually appealing behaviour, including gentle sparring displays between males.

Do Blood Red Tetras need RO water?

In most cases, yes. Unless you are fortunate enough to have extremely soft, acidic tap water – which is uncommon in the United Kingdom and much of Europe – you will need to use reverse osmosis (RO) water to achieve the very low hardness and pH this species requires. RO units are widely available and are a worthwhile investment for anyone serious about keeping blackwater species. Some aquarists also use collected rainwater, though this should be filtered and tested before use.

Are Blood Red Tetras suitable for beginners?

No. While they are peaceful and undemanding in terms of behaviour, their strict water chemistry requirements make them a poor choice for newcomers to the hobby. Maintaining stable, extremely soft, acidic water requires experience, appropriate equipment (such as an RO unit), and diligent monitoring. Beginners would be better served starting with more adaptable species and working up to blackwater specialists like Brittanichthys axelrodi once they have gained confidence and experience with water chemistry management.

Related Guides

• Cardinal Tetra Care Guide – Another iconic Rio Negro species with overlapping habitat requirements and excellent compatibility with Blood Red Tetras.
• Green Neon Tetra Care Guide – A closely related blackwater tetra that thrives in the same soft, acidic conditions.
• Chocolate Gourami Care Guide – A Southeast Asian soft-water specialist with similarly exacting water chemistry needs.
• Apistogramma elizabethae Care Guide – A blackwater dwarf cichlid that makes an excellent centrepiece fish in a Rio Negro biotope alongside Blood Red Tetras.
• Dwarf Pencilfish Care Guide – A tiny, peaceful characin from similar habitats that is well suited to blackwater community setups.