Guide
How to raise fry: water quality, food and stress
The three pillars of fry survival - and how to get them right.
- Water quality first: ammonia and nitrite must stay at zero. Fry have no tolerance for either - even trace amounts kill them faster than adults.
- Small, frequent water changes: 10-15% every two to three days beats a large weekly change. Match temperature exactly and always dechlorinate.
- Feed little and often: 3-5 tiny meals a day. What they don't eat in two minutes comes out immediately - rotting food is the fastest way to spike ammonia in a small tank.
- Match food size to mouth size: livebearer fry can take crushed flake from day one; many egg-layer fry need infusoria or microworms first.
- Separate them or hide them: adults - including parents - will eat fry. A dedicated fry tank gives the best survival rates; dense planting is the fallback in a community setup.
- Cover filter intakes: a standard intake will suck up fry. Sponge filters or intake guards are non-negotiable.
Why fry die - and what's actually behind it
Most fry deaths aren't bad luck. They follow a short list of causes: ammonia poisoning from an uncycled or overfed tank, temperature shock from careless water changes, starvation because the food was too large or too infrequent, and predation by tankmates. Fix those four things and your survival rate will improve dramatically.
The mistake most guides make is treating fry care as a scaled-down version of adult fish care. It isn't. Fry are physiologically more fragile - their gill tissue is still developing, their immune systems are immature, and they have almost no energy reserves to buffer against stress. A parameter swing that an adult guppy shrugs off can wipe out a whole batch of two-day-old fry within hours.
Water quality for fry
The single most important thing you can do is keep ammonia and nitrite at zero, always. This isn't a target to aim for - it's a hard requirement. If you're seeing any ammonia reading at all, your fry are under stress even if they look fine.
Nitrate is more forgiving but still matters. Keep it under 20 ppm in a fry tank, ideally under 10 ppm for the first few weeks. In practice, frequent small water changes are your main tool for keeping nitrate down.
Water changes done right
Do 10-15% every two to three days rather than a large change once a week. The logic is simple: fry can't handle sudden shifts in temperature, pH, or hardness the way adults can. A 40% water change on a fry tank - even with perfectly matched water - creates a parameter gradient as the new water mixes in. Small changes avoid that entirely.
Before every water change: check the temperature of your replacement water with a thermometer, not your hand. Match it within 1°C (2°F) of the tank. Always dechlorinate. If your tap water is heavily chloraminated, use a dechlorinator that neutralises both chlorine and chloramine - standard sodium thiosulfate won't handle chloramine fully.
Using a gravel vacuum directly in a fry tank is a good way to accidentally suck up fry, especially small egg-layer fry that are nearly transparent. Use a piece of airline tubing as a siphon instead - the narrow bore gives you much better control.
The cycling problem
A fry tank only works if it's cycled. An uncycled tank will produce ammonia spikes within 24-48 hours of feeding, and that's a death sentence for young fry. If you're setting up a fry tank in a hurry, use filter media or substrate from an established tank to seed the cycle - it won't be instant, but it gives you a running start. See our guide on how to cycle a fish tank if you're not sure where to start.
Setting up a brand-new fry tank the day the fry arrive. Without established beneficial bacteria, ammonia climbs fast - especially when you're feeding 4-5 times a day. Always have cycled media ready before you need it, not after.
Temperature
Keep fry at the species-appropriate temperature, typically the warmer end of the adult range. For most livebearers - guppies, platies, mollies - that means 24-27°C (75-81°F). For many tetras and corydoras fry, 24-26°C (75-79°F) is appropriate. Warmer water speeds metabolism and growth, but it also speeds up bacterial activity, so water quality management becomes more urgent.
Feeding fry
Fry have two problems adults don't: their mouths are tiny, and they burn through energy fast. Feed the wrong size food and they'll starve even with a full tank of it. Feed too much and you'll foul the water.
Livebearer fry
Guppy, platy, molly, and swordtail fry are born relatively large and fully formed. They can take finely crushed flake food or powdered fry food from day one. Baby brine shrimp (Artemia nauplii) are excellent - high protein, irresistible to fry, and any uneaten shrimp stay alive in the water briefly rather than fouling it immediately. Feed 3-5 small meals daily. Each meal should be gone within two minutes.
Egg-layer fry
This is where most people struggle. Tetras, rasboras, cichlids, corydoras, and most other egg-layers produce fry that are much smaller at hatching than livebearer fry. Many are so small that even finely crushed flake is too large for the first week or two.
The first food for these fry is usually infusoria - microscopic organisms you can culture by leaving a small amount of lettuce or cucumber in a jar of tank water in a warm spot for a few days. Liquid fry food works as a commercial alternative. Microworms (Panagrellus redivivus) are a good next step: easy to culture, appropriately sized, and nutritious. Once fry reach around 1 cm, baby brine shrimp become practical.
Start a microworm culture before your fish spawn, not after. They take 3-5 days to establish and you'll want them ready from day one. A small plastic container, oat paste, and a pinch of yeast is all it takes.
How much and how often
Three to five times a day is the standard recommendation, and it's right. Fry have limited stomach capacity and high energy demands - one large feeding a day leaves them hungry for most of it. Spread meals out as much as your schedule allows.
After each feeding, remove uneaten food within 2-3 minutes. In a fry tank with minimal filtration, decaying food produces ammonia faster than in a mature community tank. This is one place where the advice really is that strict.
In a small, lightly filtered tank, a single overfeeding can produce detectable ammonia within 48 hours - often sooner at warmer temperatures.
Protecting fry from stress and predation
The predation reality
In a community tank, fry are food. That includes the parents in most species - livebearers show almost no parental instinct and will eat their own young without hesitation. Even peaceful community fish like neon tetras will pick off fry small enough to fit in their mouths. The only reliable way to raise high numbers of fry is in a separate tank.
Dense planting with java moss, guppy grass (Najas guadalupensis), or hornwort (Ceratophyllum demersum) gives fry somewhere to hide in a community setup, and you will get some survivors. But if survival rate matters to you - either because you're selling fry, keeping a specific breeding project, or working with a species you've worked hard to breed - a dedicated fry tank is the only real answer.
Breeding boxes sold as "fry savers" are a compromise at best. They keep adults out, but they restrict water flow, concentrate waste, and stress fry with confined space. They're useful for the first day or two if you're caught unprepared, but a proper separate tank with a sponge filter is better in every measurable way.
Filtration and flow
Standard hang-on-back and canister filters create suction that will pull fry straight into the intake. The fix is straightforward: cover the intake with a sponge pre-filter or fine mesh. In a dedicated fry tank, a sponge filter is often the cleanest solution - gentle flow, biological filtration, and zero intake risk. The sponge itself also becomes a grazing surface where infusoria naturally colonise, which is a bonus for small egg-layer fry.
Keep flow gentle overall. Strong circulation stresses fry and makes it harder for them to reach food. In a fry tank, you're not trying to replicate river conditions - calm, well-oxygenated water is what you want.
Light, noise, and handling
Sudden bright light or loud vibrations cause fry to scatter and can trigger stress responses that compromise immune function over time. Keep the fry tank somewhere stable - not next to a speaker, not under a window with direct sunlight. A consistent photoperiod (12 hours light, 12 hours dark) is better than irregular switching.
Minimise handling. If you need to move fry, use a cup or container rather than a net - nets are stressful and can damage fragile fins. When transferring between tanks, float the container to equalise temperature before releasing.
Fry tank setup: what actually matters
You don't need a large tank. A 20-40 litre (5-10 gallon) setup is workable for most species. What you need is: a cycled sponge filter, a reliable heater, a thermometer, and bare bottom or fine substrate (bare bottom makes cleaning much easier). That's it. Fry tanks don't need elaborate decoration - they need clean water and accessible food.
Some keepers add a small amount of java moss or spawning mop to give fry cover and a grazing surface. It's worth doing if you have it available, but don't delay setting up the tank waiting for plants.
If you're tracking multiple tanks, spawning dates, and feeding schedules, App-aquatic lets you log parameters and set reminders across tanks - useful when you're running a fry tank and a community tank side by side.
Keep a spare air-driven sponge filter running in your main tank at all times. When you need a fry tank quickly, that sponge is already colonised with beneficial bacteria - instant cycling with zero preparation time.
Growth rate and moving fry on
Fry growth varies significantly by species and water temperature, but most livebearer fry are large enough to join a community tank at 4-6 weeks, once they're big enough that adults can't swallow them. Egg-layer fry typically take longer - 6-12 weeks depending on species.
Don't rush this. A fry that gets moved to the community tank too early is a fry that gets eaten. The benchmark isn't a number of weeks - it's body size relative to the smallest adult mouth in the tank.
Feed a varied diet as fry grow: rotating between live or frozen foods and dry food produces better growth and stronger colouration than a single food source. Newly hatched brine shrimp, daphnia, and micro worms alongside a quality fry powder will get most species to juvenile size faster than dry food alone.
Switching to adult food too early. Crushed flake is fine as a supplement, but fry need high-protein live or frozen food throughout most of their early development. Switching entirely to dry food at two weeks old stunts growth and increases mortality.
Species-specific notes
Cichlid fry (angelfish, rams, apistogrammas) often receive genuine parental care - both parents may guard and herd fry actively. In a species tank, you can sometimes leave fry with parents successfully. In a community tank, remove them. Even cichlid parents under stress will eat their own fry.
Corydoras fry are tiny at hatching and need infusoria or commercial liquid fry food for the first week. They're bottom-oriented from the start, so ensure food reaches the substrate - fry powder that floats is wasted on corydoras.
Betta fry hatch from bubble nests and are initially cared for by the male. Remove the female immediately after spawning. The male guards fry for the first few days - remove him once fry are free-swimming, or he'll start eating them.
For species-specific water parameter targets, see our guide on aquarium water parameters.
Track your parameters and get care reminders with App-aquatic.
Get the free appHow long until fry can go in the main tank?
It depends on species and the size of your adult fish. The rule of thumb is: when fry are too large to fit in the smallest adult's mouth. For guppies and platies with other small community fish, that's typically 4-6 weeks. For egg-layer fry with larger tankmates, it can be 8-12 weeks. Don't go by weeks alone - look at the fry's size relative to the adults.
My fry keep dying but my water tests look fine. What am I missing?
A few possibilities. First, check that your test kit is accurate - liquid reagent kits are more reliable than test strips for detecting low-level ammonia. Second, look at temperature consistency: a heater that swings 2-3°C overnight will stress fry even if average readings look acceptable. Third, check for starvation - fry that aren't eating enough often look healthy until they suddenly die. Finally, consider whether filter intake suction might be pulling fry in without you noticing.
Do I need a heater in the fry tank?
Yes, for tropical species. Room temperature in most homes fluctuates more than a stable aquarium heater does, and fry are sensitive to temperature swings. Even if your room sits at the right average temperature, overnight drops can be enough to cause losses. A small adjustable heater is worth the investment.
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