Red Line Lizard Tetra (Iguanodectes sp.)

Meet the Red Line Lizard Tetra

The Red Line Lizard Tetra is a slim, torpedo-shaped tetra from the blackwater and clearwater streams of the Amazon and Orinoco. Its body is silvery and almost transparent, with a fine red stripe running from just behind the eye all the way to the base of the tail. A group cruising together just below the surface looks more like a shoal of tiny pike than a typical tetra, and that long, narrow shape is where the "lizard" in its name comes from.

According to Seriously Fish, the genus name Iguanodectes joins the Taino word for lizard, iwana, with the Greek word for biter. The genus contains fewer than ten species and belongs to a small family, Iguanodectidae. It is a quiet, peaceful fish for keepers who can offer soft, acidic water and a mature, stable tank.

Read the names on the label

Tell it apart from similar fish

Seriously Fish gives a simple test between the two species sold under this name: in I. geisleri a thin black stripe runs along the body below the red one, while in I. adujai it is missing. There are finer differences too, such as the position of the dorsal and anal fins and the number of anal-fin rays (20–25 in I. geisleri, 28–34 in I. adujai), but they are hard to see in a living fish. The green line lizard tetra, I. spilurus, is a much larger relative that reaches 102 mm standard length and has no red stripe.

Do not confuse this fish with the Red-Line Tetra (Hyphessobrycon amapaensis), a deeper-bodied tetra that also carries a red stripe but belongs to a different family and has different needs.

Check what one purchase brings

One purchase quantity supplies one live Red Line Lizard Tetra. The fish are not sexed at sale. Our photographs show the form of the fish; colour depth varies between individuals and is often paler for a few days after a move. Males show their best red when they are settled and competing with each other.

Plan the group before you order

• Group size: Seriously Fish recommends a mixed group of at least 8–10, kept with other schooling fish for security. OATA's general minimum for tetras is five, but this species does better in the larger group.
• Sex ratio: Seriously Fish suggests more females than males so no single fish is harassed. A larger group gives you the best chance of a good mix.
• Size: FishBase and Seriously Fish both give 55 mm standard length for I. geisleri, which leaves out the tail.
• Timing: the tank must be mature before the fish arrive. Read our delivery and live arrival details and plan the order for a day when you are home.

Plan the aquarium around a long, slim swimmer

Red Line Lizard Tetras swim mostly in the upper and middle water and need length to move as a group. Seriously Fish gives a minimum base of 120 × 30 cm, about 108 litres. That is larger than many people expect for a fish of this size, but the group is active and spreads out along the tank.

Use a three-step space check

1. Measure the base. Aim for at least 120 cm long and 30 cm deep front to back. Our tank volume calculator turns your measurements into litres.
2. Count the group. Allow for eight to ten of these fish before anything else.
3. Add the companions. Add other schooling fish and a few bottom dwellers only after the group is planned, and keep the total modest so the water stays clean.

Arrange sand, wood and leaf litter

In the wild this fish lives in small tributaries with thick overhanging vegetation, fallen branches, roots and leaf litter. Seriously Fish describes the water as acidic, low in carbonate hardness and conductivity, and stained brown by humic substances. In the upper Orinoco it is also found in morichales: clear streams with sandy bottoms and dense plants.

Divide the tank by function

Seriously Fish says a sandy floor with driftwood suits it best, that it also does well in a planted tank, and that natural peat is unnecessary. Dried leaf litter adds cover and a source of microbial food.

Set soft, acidic water values

This is the part of care that decides success. The published values for this fish are softer and more acidic than OATA's general tetra range, and they come from a species-specific source, so we follow them.

Compare the published values

OATA itself notes that wild-caught fish may need different values from its general range. Ammonia and nitrite must stay at zero, and nitrate as low as you can keep it.

Most UK tap water is harder than 5°dH, especially in the south and east of England. Blend tap water with reverse osmosis water to reach the range, keep the mix the same at every water change, and let wood and leaves lower the pH gently rather than adding chemicals. Our water chemistry guide and the guide to pH for tropical fish explain how. The water matcher shows which other fish suit the same water.

Build filtration for a sensitive fish

Seriously Fish warns that this species is sensitive to poor water and should never go into a biologically immature aquarium. That means a fully cycled filter, steady readings for several weeks, and no shortcuts. If you are setting up from scratch, our explanation of the nitrogen cycle shows how long it takes. Seriously Fish suggests a filter turning the water over four to five times per hour; our aquarium filtration guide helps you choose one.

Follow a weekly maintenance sequence

1. Test ammonia, nitrite, nitrate, pH and hardness at least weekly, as OATA advises.
2. Change about 25% of the water every week with water already blended to the same hardness.
3. Siphon debris from the sand, leaving some leaf litter in place and topping it up as it breaks down.
4. Rinse filter media only in old tank water.
5. Check the heater and thermometer; stay within 23–26°C.

Feed a varied, measured diet

Seriously Fish believes the species is omnivorous in nature, feeding on small invertebrates, crustaceans, filamentous algae and fallen fruit. It accepts dried foods in the aquarium but does best on a varied menu that includes bloodworm, mosquito larvae, Daphnia and Moina.

Build one measured meal

• How often: once a day, as OATA advises, giving only what the group clears within a few minutes.
• The base: a quality small flake or micro-granule that floats or sinks slowly, since this fish feeds high in the water.
• The extras: frozen daphnia and live mosquito larvae several times a week, which also bring out the males' colour.
• The check: remove leftovers; uneaten food in soft, acidic water degrades quickly.

Introduce the group to a mature aquarium

Turn the lights off before you start. OATA's method is to float the bag for about 10 minutes, then add small amounts of tank water over up to 20 minutes. Practical Fishkeeping warns that after a long journey ammonia rises in the bag and the pH drops, so once the bag is open the fish should go into the tank promptly, or you should add an ammonia detoxifier. Use a net and leave the transport water behind.

Use a first-week checkpoint plan

Practical Fishkeeping advises a quarantine or hospital tank whatever the source of your fish, and it is a sensible step for a sensitive species going into an established tank.

Watch the group and act on changes

A healthy group swims near the surface with fins open, and males flash red on their fins when they spar. OATA lists warning signs: hanging at the surface or sitting on the bottom, fast breathing, clamped fins, flicking, white spots, fluffy growths, colour much paler or darker than usual, weight loss and poor appetite.

Pair each observation with an action

Choose calm tank mates for soft water

Seriously Fish describes the species as generally peaceful and suggests similarly sized characids, hatchetfish, pencilfish, small catfish of the corydoras and loricariid families, and non-predatory small to medium cichlids as tank mates. Males compete among themselves, which is display rather than real fighting, and they show their best colour while doing it.

Screen each tank mate

Find more soft-water companions in the South American tetra category, our tetras page and our range of schooling fish.

Read the signs of sex and breeding

Seriously Fish notes that adult males carry red markings on the pectoral, pelvic, anal and tail fins, which females lack. In the related green line lizard tetra, nuptial males also develop small hooks on the front rays of the anal fin. Breeding has not been described in the sources we checked, and OATA notes that tetras rarely breed successfully in a community tank. Treat this as a display and community fish, not a breeding project.

Answer Red Line Lizard Tetra buying questions

Is the Red Line Lizard Tetra the same fish as the Red-Line Tetra?

No. The Red-Line Tetra sold on our site is Hyphessobrycon amapaensis, a deeper-bodied tetra from a different family. The Red Line Lizard Tetra is a slim Iguanodectes, a pike-shaped fish with a red stripe from the eye to the tail.

Which species is it?

We label it Iguanodectes sp. Seriously Fish gives the name red line lizard tetra to I. geisleri and notes that I. adujai is sold under the same name. In I. geisleri a thin black stripe runs below the red one; in I. adujai it is absent.

Why is it called a lizard tetra?

The genus name Iguanodectes joins the Taino word for lizard, iwana, with the Greek for biter, as Seriously Fish explains. It describes the long, slender, lizard-like body.

How big does it grow?

Seriously Fish and FishBase both give a maximum of 55 mm standard length for I. geisleri, measured without the tail. With the tail, expect a slim fish of roughly 6–7 cm.

How many should I keep?

Seriously Fish recommends a mixed group of at least 8–10, with more females than males, kept alongside other schooling fish. One or two kept alone will hide and lose colour.

What water does it need?

Soft, acidic water. Seriously Fish gives 20–26°C, pH 5.0–7.0 and 1–5°dH. Most UK tap water is harder than this and needs blending with reverse osmosis water.

How can I tell males from females?

Adult males have red markings on the pectoral, pelvic, anal and tail fins, which females lack, according to Seriously Fish. Males also show their strongest colour when competing with each other.

Can it go into a newly set-up tank?

No. Seriously Fish says it is sensitive to poor water and should not go into a biologically immature aquarium. Wait until the filter is fully cycled and the tank has run steadily for some weeks.

Consult the care references

• Seriously Fish: Iguanodectes geisleri — trade name, range, habitat, size, tank size, water values, diet, behaviour, sexing and how to tell it from I. adujai.
• FishBase: Iguanodectes geisleri — family, distribution, maximum size and conservation status.
• Seriously Fish: Iguanodectes spilurus — the meaning of the genus name, the larger green line relative and its breeding dress.
• OATA: How to look after tetras and pencilfish — general water values, maintenance, feeding, acclimatisation and health signs.
• Practical Fishkeeping: How to acclimatise your new fish or shrimp — transport risks and quarantine.