Asking what fry eat has a different answer every few days, which is what makes the first weeks both fascinating and easy to get wrong. A newborn guppy can tackle powdered food almost immediately, while a newly hatched fry of an egg-scattering species may be too small for anything but the microscopic organisms drifting in mature tank water. The thread running through every case is mouth size and feeding frequency: fry eat what fits, they eat it often, and the menu has to keep pace with growth that is visible from one week to the next.
The first days: yolk, then microscopic food
Most fry do not eat at all on day one. They live off the yolk sac they hatched with, resting on the bottom or clinging to plants and glass while it absorbs, and food added during this stage only fouls the water. Once they are free-swimming and the yolk is gone, the smallest species need infusoria: the clouds of microscopic life that grow in water cultured with a piece of lettuce or banana peel, or the green water rich in tiny algae that many breeders maintain for exactly this purpose. Liquid fry food from a tube serves the same stage in a pinch. Egg-layer fry typically spend a few days at this microscopic level, while livebearer fry, born larger, often skip it entirely.
The growth engine: baby brine shrimp and microworms
As soon as fry can manage them, newly hatched baby brine shrimp become the single best food of the first weeks. They are the right size, they wriggle and trigger hunting behaviour, and their nutritional profile drives the fast, even growth breeders want; bellies visibly tinted orange after a feed are the classic sign of a good meal. Microworms and vinegar eels, cultured inexpensively at home, cover the same stage and stay alive in the water column longer, which suits fry that feed lower down. Powdered commercial fry foods and finely ground spirulina fill the gaps between live feeds and matter more as the weeks pass, but fry raised with live food in the rotation almost always grow more uniformly than fry on powder alone.
Frequency, quantity and clean water
Fry feeding is a rhythm problem as much as a menu problem. Small stomachs and fast metabolisms mean several tiny feeds spread through the day rather than one or two large ones, and at each feed the fry should clear the food within minutes. Everything uneaten becomes pollution in a small rearing tank, so the feeding plan has to travel with a cleaning plan: siphon the bottom gently each day, top up with water matched to the tank, and keep filtration gentle enough not to exhaust or trap tiny fish, with a sponge filter the usual choice. Crowded rearing tanks benefit from moving the largest fry on as size differences appear, because the biggest fry eat first at every single feed.
Graduation and the signs to watch
By the end of the first few weeks most fry can handle finely crushed ordinary flake, small frozen foods chopped to a paste, and daphnia sized to their mouths, and from there the menu simply widens toward the adult diet of their species. Judge progress by growth and behaviour rather than the calendar: a thriving batch of fry is active, even in size, and always ready at the surface or midwater when food arrives. Fry that grow at wildly different rates, develop bent spines in numbers, or show a sudden die-off are signalling problems with food quality, water, or both, and those are worth investigating quickly rather than feeding through. Logging hatch dates, food changes and who gave which feed in KinsLe means Care Teams can keep the several-times-daily routine covered even when the breeder is out for the day.