The basics of keeping fish alive

Fish survive in an aquarium when the water stays clean, the temperature stays steady, and they get fed the right amount. That means you need three things working together: a filter to remove waste, a heater to keep the water warm (for most fish), and a feeding routine you stick to. The filter is the most important—it breaks down ammonia from fish waste into less toxic compounds, and without it the tank becomes poisonous within days.

The water itself needs to cycle before you add fish. This means letting the filter run for one to two weeks so beneficial bacteria can grow on the filter media and tank surfaces. These bacteria eat the ammonia and nitrite that would kill your fish. If you add fish before the cycle is complete, they will likely die. You can speed this up by adding a bacterial starter product or by moving some filter media from an established tank, but you still need to wait at least a few days and test the water before adding fish.

Key Takeaways

  • A working filter is essential—it removes ammonia and nitrite that build up from fish waste and would poison the tank within days.
  • Let your tank cycle for one to two weeks before adding fish so beneficial bacteria can establish themselves on the filter and surfaces.
  • Feed fish once or twice a day, only as much as they can eat in two to three minutes, because uneaten food rots and fouls the water.
  • Test water temperature, ammonia, and nitrite weekly during the first month, then monthly after that, because invisible problems kill fish before you see symptoms.
  • Do a partial water change of 20 to 30 percent every one to two weeks to remove waste that the filter cannot break down completely.

Setting the water temperature and keeping it stable

Most tropical fish need water between 75 and 80 degrees Fahrenheit. Goldfish and some other coldwater fish do fine at 65 to 72 degrees and do not need a heater. Check what temperature your specific fish species needs before you buy them, because temperature stress weakens their immune system and causes disease.

Use a submersible heater rated for your tank size—a 50-watt heater works for a 10-gallon tank, a 100-watt for a 20-gallon, and so on. Pair it with a thermometer so you can see the actual temperature. The heater will cycle on and off to maintain the set point. Place the heater where water can flow around it, not in a corner where it sits in still water, because that can cause hot spots. Check the temperature at the same time each day for the first week to make sure it is stable.

Feeding fish the right amount and frequency

Feed fish once or twice a day, depending on the species. Most fish do well on one feeding a day, but some (like fry or very small fish) need two. The rule is straightforward: feed only what they can eat in two to three minutes. Any food that sinks to the bottom and sits there will rot, foul the water, and spike ammonia. Overfeeding is the most common mistake new aquarists make, and it kills more fish than underfeeding.

Use a pinch of quality flake food or pellets—not a handful. Watch the fish eat. If food is still falling to the bottom after two minutes, you gave too much. Remove uneaten food with a net or siphon. Different fish eat different foods: some need sinking pellets, some need floating flakes, and some need live or frozen food like bloodworms or brine shrimp. Research your specific fish before you buy them so you know what to feed.

Testing water and understanding what the numbers mean

Water testing tells you whether the tank is safe before fish show signs of stress. You need to test for ammonia, nitrite, and nitrate. Ammonia and nitrite should always be zero—any reading above zero means the tank is not cycled or the filter is failing. Nitrate will climb over time and should stay below 40 parts per million (ppm), though lower is better. If nitrate climbs above 40, do a water change.

Buy a liquid test kit (brands like API Master Test Kit are standard) rather than test strips, because strips are less accurate. Test weekly during the first month while the tank is cycling, then monthly after that. If you see ammonia or nitrite spike, do a 25 to 50 percent water change when ready and check that the filter is running and not clogged. If the problem does not go away in a few days, something is wrong with the filter or too many fish are in the tank.

Doing water changes to remove waste the filter cannot handle

Even a good filter cannot remove all the waste fish produce. Nitrate builds up over time and will poison fish if it gets too high. A partial water change removes nitrate and other dissolved waste. Do a 20 to 30 percent change every one to two weeks in an established tank. During the first month while the tank is cycling, do a 25 percent change every few days if ammonia or nitrite is above zero.

Use a siphon or gravel vacuum to remove water from the bottom of the tank, where waste settles. This also cleans the gravel. Replace the water with fresh water that has been treated with dechlorinator if your tap water is chlorinated (most municipal water is). Let the new water reach room temperature before adding it, or add it slowly so you do not shock the fish with a sudden temperature change. Never do a 100 percent water change unless the tank has crashed—it kills the beneficial bacteria and restarts the cycle.

Cleaning the filter without destroying the bacteria

The filter media (sponge, cartridge, or floss) traps waste and houses the beneficial bacteria. Clean it when water flow slows, usually every two to four weeks. Rinse the media in old tank water that you removed during a water change, not in tap water. Tap water chlorine kills the bacteria you need. Squeeze the media gently until the water runs clear, then put it back in the filter. This removes trapped waste while keeping the bacteria alive.

If the filter housing itself is clogged with algae or slime, rinse that in old tank water too. Never replace all the media at once unless the filter has failed completely. If you must replace media, do it in stages—replace half, wait a week, then replace the rest. This keeps enough bacteria alive to prevent an ammonia spike.

Recognizing signs of trouble before fish die

Healthy fish are active, eat eagerly, and have bright colors. Fish under stress become lethargic, hide, stop eating, or develop spots and sores. Common problems are ammonia or nitrite poisoning (fish gasp at the surface or act drunk), ich (white spots on the body), fin rot (ragged fins), and bacterial infections (cloudy eyes or open sores). Most of these start with water quality—test when ready if fish act sick.

If ammonia or nitrite is above zero, do a 50 percent water change right away and check the filter. If the water is clean but fish are still sick, the problem is usually overcrowding, wrong temperature, or a contagious disease. Isolate sick fish in a separate small tank if possible so disease does not spread. If you are not sure what is wrong, describe the symptoms to an experienced aquarist at a local fish store—they see these problems every day and can usually tell you what to do.

Frequently Asked Questions

How long does it take for a tank to be ready for fish?

One to two weeks if you do nothing but run the filter, or three to five days if you add a bacterial starter product. The only way to know for sure is to test ammonia and nitrite—when both read zero for three days in a row, the tank is cycled. Adding fish before that will likely kill them.

Can I use tap water straight from the faucet?

Only if your tap water has no chlorine or chloramine. Most municipal water does, so you need a dechlorinator. Add it to the water before it goes in the tank. Some dechlorinators also bind heavy metals. If you are not sure whether your water is treated, call your water utility or buy a dechlorinator—they are cheap and safe to use.

What should I do if a fish dies?

Remove it when ready so it does not rot and foul the water. Test ammonia and nitrite to see if something went wrong with the tank. If the water is clean, the fish may have been sick or stressed. If ammonia or nitrite is high, do a water change and check the filter. Do not add a new fish right away—wait a few days to make sure the tank is stable.

How many fish can I put in my tank?

It depends on the fish species and tank size. A common rule is one inch of fish per gallon of water, but that is rough—some fish need more space than others. Research your specific fish before you buy them. Overcrowding is the fastest way to crash a tank because the filter cannot keep up with the waste.

Do I need to do anything during vacation?

Fish can go one to two weeks without food if the tank is established and stable. Do a water change before you leave, make sure the heater and filter are working, and do not feed them before you go. If you will be gone longer than two weeks, ask someone to feed them once a day or use an automatic feeder. Never overfeed before you leave—that fouls the water faster than the fish starving.