Black Skirt Tetra Feeding: Diet, Food Size, and How Often to Feed

The black skirt tetra (Gymnocorymbus ternetzi), also sold as the black widow tetra, is an easy fish to please at mealtime. It is an omnivore, it eats the same dried food as most of a community tank, and it does not gorge itself. What it asks of a keeper is a little live or frozen food now and then and a portion sized to the school.

Feeding Schedule by Life Stage

Young black skirts get small, frequent meals, and the adults settle on one a day.

Adult black skirts eat their own eggs whenever they get the chance, so they come out of the spawning tank the moment eggs are seen. The eggs hatch in 18 to 36 hours, and the fry swim free a few days later.

For the adults, one meal daily is plenty, and the school should be fed on at least five days of the week. Published maintenance rations for pet fish run from 0.5 to 1% of body weight a day up to about 3%. Start at the low end with this species, since black skirts are not greedy eaters and do not overeat. For a school of 12 two-inch fish, either end of that range is a light pinch.

Flake fools the eye in the other direction. It weighs so little that the amount needed to feed a school can be far bigger than it looks, so weighing a day's ration once, to see what it looks like, guards against underfeeding.

What Black Skirts Eat, in the Wild and in the Tank

In the slow creeks of the Rio Paraguay and Guaporé basins, a black skirt lives mostly as a small predator. Worms, little crustaceans and insects make up the bulk of its diet. It is not a selective feeder, though, and it swallows modest amounts of algae and plant matter along with its prey. That mix, mostly animal food with a small share of plants, is the pattern a tank diet copies.

The black skirts sold in the trade are all bred in captivity, and that long history in tanks has left them easy to feed. A good flake or granule food makes a sound daily staple, and a slow-sinking micro-pellet works in the same role. Omnivorous fish have taken to captivity better than most, and this species shows why: there is very little it refuses.

A black skirt reaches 2 to 2.5 inches (5 to 6 cm) in a home tank, and up to 3 inches at most. That is big enough to take flake and granules as a staple without the fine crushing an ember tetra at 0.8 inch needs. Live and frozen extras should still be small items, the size of daphnia or bloodworm, since those match the small prey the fish catches in the wild.

Small live and frozen foods belong in the routine on a regular basis, alongside the dried staple. Daphnia and bloodworm are the standard choices, and both are small invertebrates of the kind that make up the wild diet. Daphnia has a known downside: it and cyclops are common intermediate hosts of Camallanus, a parasitic worm, and tank fish can end up heavily infected when the crustaceans that carry it are in the food.

Dried food has a shelf life. An opened container kept at room temperature goes in the trash 2 months after opening, and commercial food kept in the freezer after 6 months. Frozen bloodworm, brine shrimp and the like keep a year, and food that has thawed is never refrozen.

Foods to Avoid

• Wild-caught live food. Daphnia or worms collected outdoors can bring disease into the tank. Cultured live food is a better source of nutrition, and even that needs care so it does not keep an infection going.
• Fatty, starchy or rancid diets. Fatty liver disease is a common problem in captive fish, and a high share of carbohydrate, a high fat content and rancid food are among its causes, along with starvation.

Why Black Skirts Stop Eating

1. Cold water. Appetite follows the thermometer. Held for two weeks at four temperatures from 68 to 90°F (20 to 32°C), black skirts ate more as the water got warmer. White skirts, the pale form of the same species, cut their feeding and activity at about 63°F (17°C) and stopped eating as the water approached 54°F (12°C), so a heater that has failed is the first thing to rule out. A school in a tank kept near the cool end of the black skirt's 68 to 79°F range can be expected to take less food than one near the top, and the portion should shrink to match.
2. Already full. Because black skirts do not overeat, food the school does not want simply drifts down and settles. That is a sign the meal was too big, so the next one should be smaller. Leftovers that sit in the tank degrade the water for every fish in it, adding to the waste the nitrogen cycle in an established tank has to process.
3. Internal parasites. Spironucleus, an intestinal flagellate, causes weight loss and a lost appetite.

Around other species, black skirts are not known to compete hard for food, so a school seldom crowds its tankmates out of a meal.

When to Worry

Overfeeding is the commonest mistake keepers make, but the opposite happens too. A keeper fighting a water problem can cut the food back so far that the fish run a calorie deficit and start losing weight. The signs that need action:

• Fish getting thinner on a cut-back ration. That is underfeeding, and the portion goes back up.
• Weight loss with a pot belly. Capillaria, a worm found in tetras and other aquarium fish, causes both, and it needs a diagnosis before treatment.
• Worms at the vent. Camallanus shows as worms protruding from the vent, most likely in a tank fed daphnia or cyclops.

When You Go Away

A trip away calls for a person to feed the school or an automatic feeder, and either one can overfeed a species that leaves its surplus on the bottom.

For a sitter, portion out every meal in advance, for example in a labeled pill organizer with one compartment per feeding, so the amount cannot creep up. Show them once, and leave your number.

For an automatic feeder, set it up and run it for several days before you leave, with a food it dispenses well, and check that it is not jamming or overfeeding.

A black skirt will eat nearly anything sized for it, which is why the keeper's real decision is the portion, not the food.

Sources & Further Reading