Guide
My child's fish died
What to do when your son's or daughter's fish dies - practical steps to handle the tank, support your child, and fix the cause before adding new fish.
- Remove the fish immediately: a decomposing body spikes ammonia and can kill your remaining fish within hours.
- Test the water before anything else: ammonia above 0.25 ppm or any detectable nitrite means do a 25-50% water change today.
- Check the basics: is the filter running, is the heater holding temperature, and has the tank been properly cycled?
- Be honest with your child: short, calm, and blame-free. Let them be sad - it's a real loss.
- Don't replace the fish yet: find and fix the cause first, or the next fish will likely die too.
- Most fish deaths in children's tanks are preventable: the usual culprits are an uncycled tank, overfeeding, and overstocking.
First: get the fish out
Use a net or a small cup to remove the fish as soon as you find it. Don't leave it in "just until the kids say goodbye" - a decomposing fish releases ammonia fast, and if your tank is already stressed that can tip surviving fish into crisis. Once removed, wrap it in paper towel and bin it, or bury it in the garden if a small ceremony feels right for your family. Either is fine. The tank comes first.
While you're at the tank, take thirty seconds to look at every other fish. Are they swimming normally? Are any gasping at the surface, sitting on the bottom, or clamped up with their fins pressed flat against their bodies? Anything unusual means you need to act quickly on water quality - don't wait for a test result to do a partial water change if the fish look distressed.
Check the tank: water first, equipment second
This is where you find out whether you have a single loss or a bigger problem developing. Test for ammonia, nitrite, and nitrate. If you don't have a test kit, most aquarium shops will test a water sample for free - take a small bottle of tank water with you.
Ammonia and nitrite should both read 0 ppm in a healthy, cycled tank. If either is elevated, your remaining fish are already under stress even if they look okay right now. Do a 25-50% water change immediately and retest in 24 hours.
Once you've dealt with water quality, check the equipment. Run your hand along the filter outlet - is water flowing? Lift the heater guard and look at the temperature reading on a separate thermometer (heaters fail in both directions: stuck on, or stuck off). For most tropical fish you're aiming for 24-27°C (75-80°F). Goldfish and other coldwater species should be kept well below that, around 18-22°C (64-72°F) - a common mistake is keeping goldfish in a heated tropical tank.
For a full explanation of what the numbers mean and what's dangerous, see our guide to aquarium water parameters.
The most likely reason your child's fish died
Children's tanks - especially first tanks bought as gifts - follow a fairly predictable pattern. Here are the causes that account for the vast majority of losses.
The tank was never cycled
This is the single biggest killer in beginner setups. A new tank has no beneficial bacteria to process fish waste. Ammonia builds up from day one, and fish can die within days or weeks of being added. The process of establishing those bacteria colonies is called the nitrogen cycle, and skipping it is almost universal in first-time setups because pet shops rarely explain it properly at point of sale.
If your tank is less than six to eight weeks old and was set up with fish in it from the start, an uncycled tank is almost certainly what killed your fish. Read our guides on the nitrogen cycle and how to cycle a fish tank before adding any replacements.
Overfeeding
Kids feed fish. Often. Sometimes multiple times a day, sometimes pouring in a large pinch because "they look hungry." Uneaten food rots, drives up ammonia, and depletes oxygen. The correct amount for most small fish is what they can completely consume in two minutes, once or twice a day. If there's food sitting on the gravel after feeding, that's too much.
Thinking a fish that comes to the glass is begging for food. Fish come to the glass because movement triggers an instinct - they are not actually hungry. Feeding on demand is one of the fastest ways to foul a small tank.
The tank was too small or overstocked
A bowl, a 5-litre desktop tank, or a small gift aquarium creates water quality problems that are very difficult to manage. Small volumes of water swing in temperature, accumulate waste faster, and leave almost no margin for error. Many first children's tanks are sold with fish that will outgrow them or that produce waste loads the filtration can't handle.
Use the free stocking calculator to check whether your tank is appropriately stocked for its size and filtration.
Disease
If the fish died within a day or two of being brought home, it may have been sick before you bought it. Look at your other fish for white spots (ich/Ichthyophthirius multifiliis), frayed fins, unusual swimming, or fuzzy patches. If you see any of those, treat before adding anything new - disease spreads fast in a closed system.
Talking to your child
You know your child better than any guide does, so treat what follows as a framework rather than a script.
Be direct and calm. "The fish died. Sometimes fish die, even when we look after them." For younger children, that's usually enough of an explanation. Avoid vague language like "went to sleep" or "went away" - it creates confusion and can make bedtime anxiety worse for some kids.
Don't assign blame, including to yourself out loud. If overfeeding was a factor, the honest framing is "we were feeding a bit too much - now we know, and we'll do it differently." That turns it into a learning moment rather than a guilt trip for anyone.
Fish are often a child's first experience of death. That's not a bad thing - it's a genuine, low-stakes opportunity to talk about loss in a way that isn't overwhelming. You don't need to have all the answers. "I don't know exactly why it died, but we're going to figure it out" is a perfectly good thing to say.
A small ritual can help close the moment: letting your child draw the fish, saying a few words before disposing of it, or writing its name down. Whether you do any of that depends entirely on your family. Some kids want to move on fast; others need the acknowledgement. Follow their lead.
Before you buy a replacement
The urge to replace the fish quickly - especially to soften the blow for your child - is understandable, but it's the move most likely to result in a second death. Fix the underlying problem first.
Make the process of fixing the tank part of the activity. Letting your child help test the water, watch you do a water change, or research the next fish together turns a sad event into engagement with the hobby. It also sets more realistic expectations about what keeping fish actually involves.
Once water parameters are stable - ammonia and nitrite both at 0 ppm, nitrate below 20 ppm, temperature correct for the species, filter running properly - you're ready to think about new fish. Check how often to do water changes and establish a routine before adding anything new. A 20-25% weekly change is the baseline for most stocked freshwater tanks.
If you're choosing a replacement, pick something genuinely suited to your tank size and your family's maintenance reality. Our guide to the best fish for beginners gives honest options with real care requirements.
Almost always linked to an uncycled tank or ammonia spikes - both preventable with basic water testing.
Keeping on top of it going forward
The main thing that separates tanks that run well from tanks that have repeated losses is regular, brief water testing. You don't need to test daily - once a week is enough when things are stable, and after any change (new fish, medication, power cut, holiday feeding). If you want a simple way to log test results and get reminders for water changes, App-aquatic lets you track all of that in one place so nothing gets forgotten.
Keep a small stock of dechlorinator and a basic test kit (liquid tests are more reliable than strips) under the tank. Both are cheap insurance. The cost of replacing fish repeatedly vastly outweighs the cost of a £10 test kit used consistently.
Track your parameters and get care reminders with App-aquatic.
Get the free appHow long should I wait before getting a replacement fish?
Until your water is stable: ammonia and nitrite at 0 ppm, nitrate below 20 ppm, and the tank fully cycled. For a tank that wasn't cycled properly, that could mean four to eight weeks of fishless cycling. Rushing it is the most common reason the replacement fish dies too.
My child wants to do a funeral. Should I flush the fish?
Flushing is fine - it's a common choice and there's nothing harmful about it for most household plumbing. If your child wants a burial in the garden, that works too. The only thing to avoid is leaving the body in the tank for extended periods while you decide.
The fish was only bought two days ago. Did the shop sell me a sick fish?
Possibly. Fish that die within 24-72 hours of purchase were often already compromised - by stress during transport, disease, or poor conditions at the shop. Check your remaining fish carefully for signs of illness. Most reputable shops will offer a replacement or refund with a dead fish and a water sample, so it's worth asking.
Could the other fish have killed it?
Yes, if the tank contains incompatible species. Fin-nipping, territorial aggression, and size mismatches are all real causes of death. If the dead fish had torn fins or visible injuries, aggression is likely. Research compatibility before adding new fish, and check the stocking calculator to make sure the tank isn't overcrowded.
My child is devastated. Is this normal?
Completely. For many children this is their first experience of losing a pet, and the grief is real even if the fish was small and inexpensive. Don't rush them past it. Acknowledging the loss directly - "it's okay to feel sad, he was your fish" - is more helpful than trying to minimise it or immediately offer a replacement.
More guides · Why fish die suddenly · Cycle a fish tank · Overfeeding · Get the app