The basics of keeping fish alive

Fish need four things to survive: clean water, the right temperature, food, and space. Most fish die not from disease but from water that has built up ammonia and nitrite—the invisible waste products of their own bodies. A working filter removes these poisons. A heater keeps tropical fish in the range they need (usually 75 to 80 degrees Fahrenheit). Food should be small enough that they finish it in two to three minutes, once or twice a day. Tank size matters because more water dilutes waste faster, which is why a 10-gallon tank is harder to keep stable than a 40-gallon one.

The single most common mistake is overfeeding. Uneaten food rots and creates ammonia. Fish also produce more waste when overfed, which crashes water quality. A second mistake is changing too much water at once—more than 25 percent in a single day can shock the fish by changing temperature and chemistry too fast. The third is adding fish before the tank is cycled, meaning before beneficial bacteria have colonized the filter to convert ammonia into less toxic nitrate.

Key Takeaways

  • A working filter and heater are non-negotiable; they remove waste and maintain temperature, which are the two things that kill most aquarium fish.
  • Feed small amounts once or twice daily—only what the fish will eat in two to three minutes—because uneaten food and overfeeding both degrade water quality.
  • Change 20 to 25 percent of the water once a week; larger changes shock the fish and undo the work of your filter's beneficial bacteria.
  • Test water for ammonia and nitrite before adding fish, and retest weekly for the first month to catch problems before they kill anything.
  • Different fish have different needs; research the specific species before you buy, because a goldfish and a betta have almost nothing in common.

Setting up the filter and maintaining water quality

Your filter has two jobs: mechanical (trapping solid waste) and biological (growing bacteria that convert ammonia to nitrate). Most aquarium filters use a sponge or cartridge for mechanical filtration and a chamber with media—ceramic rings, lava rock, or sponge—for bacteria to colonize. The bacteria take two to four weeks to establish, a process called cycling. During this time, ammonia and nitrite will spike and then fall as the bacteria population grows.

To cycle a tank safely, add fish slowly and feed lightly, or use a fishless cycle by adding ammonia directly and testing daily until the bacteria are established. Once cycled, run the filter 24 hours a day. Clean the mechanical part (sponge or cartridge) every two weeks by rinsing it in old tank water—never tap water, which kills the bacteria. Replace cartridges only when water flow slows noticeably, usually every three to six months depending on the filter and tank load.

Test water with a liquid test kit (not strips, which are less accurate) for ammonia, nitrite, and nitrate. Ammonia and nitrite should always be zero. Nitrate will climb over time and is removed by water changes. If ammonia or nitrite appear, increase water changes to 25 percent twice a week, reduce feeding, and check that the filter is running and not clogged.

Temperature control and heater placement

Most tropical fish need water between 75 and 80 degrees Fahrenheit. Goldfish and some other coldwater species need 65 to 72 degrees and do not need a heater at all. Check the species you plan to keep before you buy the tank. A submersible heater rated for your tank size (usually 3 to 5 watts per gallon for a 20-gallon tank, less for larger tanks) should be placed where water flows past it—near the filter outlet or in a high-traffic area of the tank.

Use a separate thermometer to check temperature daily for the first week, then a few times a week after that. Heaters can fail silently or stick on, so a thermometer is your only warning. If temperature swings more than 2 degrees in a day, the heater may be failing or the room is too cold. If it climbs above 82 degrees, unplug the heater and check that it is not stuck in the on position. Most heaters last two to five years before they need replacement.

Feeding schedules and portion control

Feed once a day for most fish, twice a day only if the tank is large and the fish are young. A good rule is to feed an amount the size of the fish's eye—tiny pinches for small fish, a small pinch for medium fish. Watch the fish eat. If food hits the bottom uneaten, you gave too much. If the fish are still searching for food after three minutes, you gave too little. Adjust the next day based on what you see.

Different foods suit different fish. Flakes work for most community fish. Pellets sink and work for bottom feeders. Frozen food (thawed first) like bloodworms or brine shrimp adds variety and nutrition. Live plants in the tank also provide grazing for some species. Skip feeding one day a week—fish can go a week without food and it helps keep water quality stable.

Never feed fish food that has been open for more than six months. Old food loses nutrition and can harbor mold. Store food in a cool, dry place, not above the tank where heat breaks it down faster.

Water changes and tank maintenance

Change 20 to 25 percent of the tank water once a week. Use a siphon or gravel vacuum to remove water from the bottom, where waste settles. This removes nitrate (which builds up over time) and dilutes any toxins. Refill with dechlorinated water at the same temperature as the tank. If your tap water is very hard or has high chlorine, let it sit overnight before adding it, or use a water conditioner.

During water changes, inspect the tank for dead plants, uneaten food, or dead fish. Remove these when ready. Check that the heater and filter are still running. Wipe algae off the front glass if it blocks your view, but leave algae on the back and sides—it is harmless and fish graze on it. If algae becomes thick and green, reduce light to 8 to 10 hours a day and do not overfeed, because algae feeds on excess nutrients.

Clean the filter intake tube and outlet if they become clogged with algae or debris. Do not deep-clean the filter media itself unless water flow has slowed to a trickle—the bacteria in the media are what keep the tank stable, and aggressive cleaning kills them.

Recognizing sick fish and when to act

Healthy fish have clear eyes, smooth scales, and fins held upright. They eat eagerly and move with purpose. Sick fish may have cloudy eyes, torn fins, spots or sores, or they may hide, gasp at the surface, or refuse food. The most common causes are poor water quality (ammonia or nitrite), temperature swings, or overcrowding. Check water parameters first—if ammonia or nitrite are above zero, do a 25 percent water change when ready and retest in a few hours.

If water is clean and temperature is stable, the fish may have a bacterial or parasitic infection. Isolate the sick fish in a separate small tank (a 5-gallon bucket with a heater and filter works) to prevent spread and to treat it without medicating the whole tank. Many infections clear on their own with clean water and stable temperature. If the fish does not improve in a week, or if multiple fish show the same signs, consult a veterinarian who treats fish or a knowledgeable aquarium store.

Do not add medication to the main tank without knowing what you are treating. Many medications kill beneficial bacteria and can make things worse. Quarantine is safer and more effective.

Choosing fish that match your tank and experience level

Beginner fish are hardy, tolerate temperature swings, and do not need special food or conditions. Guppies, tetras, corydoras catfish, and platies are forgiving. Bettas are hardy but must be kept alone (males fight). Goldfish are hardy but need cold water and a large tank—a single goldfish needs at least 20 gallons, not the small bowls they are often sold in. Avoid delicate fish like discus or seahorses until you have kept a stable tank for at least six months.

Research the adult size of any fish before you buy it. A fish sold as 1 inch may grow to 4 inches. A tank that is crowded will have poor water quality no matter how good your filter is. A general rule is 1 inch of fish per gallon of water, but this varies by species and activity level. Ask at the aquarium store or look up the species online.

Do not mix fish that have different temperature needs. Do not keep aggressive fish with peaceful ones. Do not keep fish that eat smaller fish with the smaller fish. These mistakes lead to stress, injury, and death. A 20-gallon tank with five guppies and five tetras is stable and peaceful. A 20-gallon tank with a goldfish and a betta is a disaster waiting to happen.

Frequently Asked Questions

How often should I do a full water change?

Never. A full water change removes all the beneficial bacteria in the filter and crashes water quality. Stick to 20 to 25 percent weekly. The only exception is if a fish dies and the tank smells, in which case remove the dead fish, do a 50 percent change, and then resume normal 25 percent changes the following week.

Can I use tap water straight from the faucet?

Not if your tap water is heavily chlorinated. Chlorine kills fish and bacteria. Let tap water sit overnight before adding it, or use a water conditioner (a few drops per gallon). If your water is very hard or has high nitrate, a water conditioner or reverse osmosis filter helps, but for most people, letting it sit is enough.

What do I do if my fish stop eating?

First, check water temperature and test for ammonia and nitrite. If both are normal and temperature is stable, the fish may be stressed by a recent change (new tank mates, moved decoration, loud noise). Leave the fish alone for a day and try again. If it still will not eat after three days, isolate it in a separate tank with clean water and observe for other signs of illness.

How long can fish survive without a filter?

A few hours in a small tank, longer in a large one with plants. A filter is essential for any permanent setup. If your filter breaks, do 50 percent water changes twice a day until you can replace it, and reduce feeding to once every other day. Do not wait to fix it—fish will die in a day or two without filtration in a small tank.

Is it normal for fish to hide all the time?

Some fish are naturally shy and hide during the day, especially when first added to a tank. But if a fish that was active suddenly hides and will not eat, something is wrong. Check water quality and temperature first. If both are fine, the fish may be stressed by tank mates or a recent change. Give it a few days in a quiet tank before deciding it is sick.