Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Knodus borki

Blue tetra

Kissing gourami, Helostoma temminckiiKissing gourami, Helostoma temminckii
Photos by Michelle Stuart


Knodus: Greek, knodon, -ontos = knife, sword

General Notes:

For many years there was some confusion over the true identity of this fish. Often the fish went under the name of Boehlkea fredcochui. The two fish are very similar but there is a difference in dentation which sets the two apart. It is Knodus borki which is often seen for sale under the name of Boehlkea fredcochui. Knodus borki is a very lively fish which really needs a slightly larger aquarium than most fish of a similar size in order to allow it to behave naturally. As with most small tetras Knodus borki is a shoaling species and they should be kept in groups of at least five individuals. Blue tetras can be a little nippy amongst themselves and in cramped conditions this is likely to be worse. Don't risk keeping this species with delicate or nervous fish or fish which are either slow moving or have long or flowing fins. They can be kept quite safely with fish of a similar nature like danios, barbs and other small tetras. Knodus borki isn't as hardy as most small community tetras and soft acidic water is essential if they are to thrive. With good care expect a life span in captivity of 2 - 4 years.


Blue tetras will greedily accept flake food, suitably sized granular food which should be supplemented with live or frozen food like daphnia, cyclops and blood worms. Due to their nature make sure that any tank mates get their share of food.


Females are larger and more rounded than males when mature, males are slightly more brightly coloured than the females.


Breeding has been achieved by hobbyists but the blue tetra is a very difficult fish to breed. Commercial breeders use hormones to breed this fish. Try a standard set up for breeding tetras but pay particular attention to making sure the water is very soft and acidic.

Use a separate aquarium filled with soft acidic aged water and containing a small air powered sponge filter and something to act as a spawning mop such as a bunch of fine leaved plants tied in to a bunch for example. Keep the tank away from direct light and in very dim conditions because both eggs and fry of this species are light sensitive. Introduce a well conditioned pair in the evening and they will probably spawn at dawn. After spawning remove the parents to prevent them from eating all their eggs. The eggs hatch after about 30 hours but it will be another three or four days before the fry become free swimming. Add a commercially available liquid fry food to the tank about a day or so before the fry become free swimming as this will encourage the growth of infusoria which the fry can eat but don't add to much and pollute the water. Carry on using the liquid food or similar until the fry are large enough to take micro worms or newly hatched brine shrimps.

Wild status

Wild populations of Knodus borki have not been evaluated but the species isn't thought to be under any immediate threat, most Knodus borki offered for sale will have bred commercially.

Information at a glance

pH: 5.5 - 6.8
dGH: 4 - 12
Temperature: 22 - 27°C (72 - 80°F)
Diet: Omnivore
Size: 5cm (2in)
Min tank size: 100 litres
Difficulty level: Intermediate
Aquarium type: Community
Swimming level: All levels

Distribution and habitat

distribution map for Helostoma temminckii

Origin: South America: Iquitos, Peru.

Habitat: Rivers and streams


Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Characiformes
Family: Characidae
Genus: Knodus
Species: K. borki, Zarske 2008

Common name:
Blue tetra

Synonyms: None