We look at the fascinating darter tetra group from South America and, using some of your frequently asked questions, explain why they make great additions to the community tank.
What are darter tetras?
Darter tetras, or South American darters, are members of a family of characin fish called crenuchids and sit in a subfamily called the Characidiinae (characidiins) that includes nine genera and more than 70 species.
The darter tetras we see most frequently in the shops are members of the Characidium genus and this includes the bulk of the species in the group, numbering more than 50.
Unlike most tetras, which live in midwater, darters are bottom dwellers and hop around on the tank floor.
Where do they come from?
They are found in most freshwater drainages in the tropical and sub-tropical parts of the neo-tropics between eastern Panama and La Plata, Argentina. However, they’re particularly common around the Guiana Shield in coastal rivers and it’s here where most species occur.
Are they widely sold?
They are not particularly common, but specialist stores can get them in to order, although the fish won’t be correctly identified.
Are they hard to ID then?
Yes. According to Professor Paulo Buckup, who is considered the definitive authority on these tetras, the group is still poorly known and includes several new species.
Many are so similar that they can only be accurately identified by examining a corpse under a microscope, so determining the fish you have is likely to be tricky. Fortunately, they all need pretty much the same aquarium care.
Are they hard to keep?
No, they’re pretty easy and certainly within the capabilities of even the fairly new fishkeeper.
What is their habitat like?
Habitats vary, but most are found in fast flowing water and usually hover over stones, pebbles and rocks on the river bed. Some are also found in waterfalls and rapids, while a handful also occur on submerged plants. Some also live in slow flowing lowland rivers.
Studies on two species from southern Brazil found the fish in a 3-4m/10-13’ wide and 50cm/20” deep clearwater forest stream. The substrate was of sand and pebbles with riffles around the rocks. They are said to prefer stony areas and water greater than 25cm/10” deep. Do they like flowing water?
Yes, their shape and position and style of their fins are an adaptation for life in torrential water, and allow them to resist the current and hang around near the bottom from where they can pick around for food among the stones.
Some, such as the stunning green Ammocryptocharax elegans, have modified pectorals that grip the leaves of plants, allowing them to stay put in very fast currents.
Some Characidium have even been seen using their fins to grip rocks and ‘climb’ waterfalls and dam walls! Scientists reckon this allows them to spread into areas they otherwise wouldn’t reach, as waterfalls are typically considered major obstacles that prevent fish species spreading upriver.
What do they eat?
Scientists have determined that these fish mainly eat insects. Their stomach contents consist largely of chironomid midge and other fly larvae, so frozen bloodworms are the perfect food — since that’s precisely what they are!
How do they feed?
Most adopt a sit and wait technique, perching on rocks and stones in fast flowing water simply waiting for any food items to drift past them.
They will also peck among rocks and gravel for morsels. Wild fish are said to feed mainly on insects and their larvae, so readily take frozen bloodworm in captivity, as well as brineshrimp, Daphnia and Cyclops.
With time they will usually take flakes, too. Some species have even been seen ‘digging’ for food.
Are they peaceful?
Yes, they’re gregarious fish and prefer to live alongside their own kind, so always keep a group of six or so. They sometimes squabble among their own kind, but usually leave other fish alone. Safe in the community tank, they pose little threat to other tank mates.
Can they be sexed?
It depends on the species, but males of some species have darker markings on their fins than females and are usually much smaller and slimmer, or a tad more colourful.
You are unlikely to see them with the naked eye, but males of many species also develop ‘sexual hooks’ on their pelvic fins and the branched rays of their pectorals. The number of hooks and size varies from species to species and this is often a useful feature when it comes to identifying those different species.
Immature males have smaller, inconspicuous hooks — and they’re completely absent from females. They could be present only during the breeding season.
Like other tetras they are egg layers and females of some species can produce up to 5,000.
The spawning hooks are believed to play a role here, the spiny projections of bone from the fins gripping the females during the spawning embrace when it’s important for the male’s sperm to be released near to the eggs.
How to keep them
Darter tetras are easy to keep and you don’t need a large aquarium to get started. Go for a tank of around 60cm/24”. Add a substrate of sand and mix in gravel and small and large pebbles, as well as some larger water-worn rocks. The odd piece of bogwood can also look good.
Use a powerful filter to provide plenty of flow and water turbulence, as these are essential to allow the tetras to behave and feed naturally. Water quality should be as good as you can possibly get it, but these fish aren’t especially sensitive and are fairly easy to keep.
If you have an existing community tank that meets at least some of these criteria, these fish should also do very well in there, though a biotope display which mimics their natural habitat is normally the best choice for any fish.
The water should ideally be soft and acidic, and not too warm. They’ve been found in waters with a pH from 5.5-7.5, GH 5-25 and temperatures ranging from 18-27°C/64-81°F.
Just the height of laziness...
Scientists from Brazil recently discovered that one undescribed species of Characidium has formed an unusual relationship with a catfish that is both clever and downright lazy!
Rather than wasting energy by looking for food itself, the Characidium simply follows a tiny suckermouth catfish called Parotocinclus maculicauda around as it grazes the rocks for algae known as periphyton. As the catfish grubs around on the rocks, it stirs up food and the waiting Characidium simply mop up the scraps!
Top fish breeder John Rundle explains how he breeds Characidium in his tanks.
What are your breeding tanks like?
I keep mine in local water which is pH 6.5-7.0 and very soft. Some people suggested they should be kept singly or in groups of eight or more, because smaller groups can act aggressively. I have not found this so and kept four in a planted tank with small mid-swimming tetras and only saw one male Characidium chasing the other, without causing any damage.
I used a 60 x 60 x 30cm/24 x 24 x 12” tank with large pea gravel and added a few clumps of Java moss and Java fern. As these fish are found in fast running streams I placed a large sponge filter in the tank with the water outlet positioned to create a current moving surface water. The temperature was 25°C/77°F.
How did you sex your fish?
I have had a few species of Characidium but bred Banded darter characin, C. fasciatum. The males have a series of dots around the base of the dorsal fin, while females have a clear fin. Females are also fuller in breeding condition. My group comprised a male and three females.I did not feed the pair after adding them to the spawning tank.
What was spawning behaviour like?
On the second day after being added to the breeding tank the fish were active, with the male chasing a female who tried to hide in the Java moss. I did not see the spawning, but on day three I noticed the female was thinner and assumed it had taken place.
Did you see the eggs?
I could not see any but removed the parents. The tank was covered with newspaper in case the eggs, if any, were light sensitive.
After two days there on the glass and Java moss were tiny yolk sac larvae and I assumed more were in the gravel. They became free swimming two days later, but spent most of their time on the gravel rather than swimming.
On what did you feed the babies?
I used my own cultured infusoria as first food. For four days I poured in 100ml of culture water each day, then on the fifth day started using brineshrimp nauplii, which the fry started to take. I continued with the nauplii and the fry grew quickly.
At 3mm long the fry were taking crushed dry foods and Grindalworms as well as nauplii.
How big was the brood?
I was surprised at the number of offspring. I split the brood when they reached 3mm and moved half to another 60cm/24” tank to prevent them getting stunted. I raised 200 and found no shortage of enthusiasts and a dealer wanting the fish.
The darter tetra subfamily Characidiinae includes these genera: Characidium; Odontocharacidium; Melanocharacidium; Microcharacidium; Elachocharax; Ammocryotocharax; Skiotocharax; Geryichthys and Klausewitzia.
This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.