Start with water quality—it's the foundation of everything

An aquarium is a closed system, which means waste builds up fast and can poison your fish. The most important job you have is keeping the water clean and chemically balanced. This means testing the water regularly, doing partial water changes, and removing uneaten food and dead plant matter before they decay.

Most beginners underestimate how often water needs to change. A good rule is to replace 25 percent of the water every week in a tank with fish. If you have a heavily stocked tank or large fish that produce a lot of waste, you may need to change 25 to 50 percent twice a week. The water you add should be the same temperature as the tank water and treated with a dechlorinator if it comes from tap water, since chlorine harms fish.

Test your water at least once a week using a water test kit. You are looking for ammonia (should be zero), nitrite (should be zero), and nitrate (should stay below 40 parts per million). If ammonia or nitrite are present, it means the tank's beneficial bacteria are not keeping up, and you need to do a water change when ready and reduce feeding.

Key Takeaways

  • Change 25 percent of the water every week, or more often if the tank is crowded or has large waste-producing fish.
  • Test ammonia, nitrite, and nitrate levels weekly using a water test kit, because these invisible chemicals kill fish even when the water looks clear.
  • Feed only what your fish can eat in two to three minutes, once or twice a day, because uneaten food rots and poisons the tank.
  • Clean the filter according to the manufacturer's instructions, usually every two to four weeks, but rinse it in old tank water to preserve the bacteria that clean the water.
  • Check the heater, air pump, and filter every day to make sure they are running, because equipment failure can kill fish in hours.

Feeding: the easiest way to wreck a tank

Overfeeding is the single most common mistake in aquarium keeping. Fish do not need much food, and leftover food sinks to the bottom, rots, and creates ammonia and nitrite that poison the water. Feed your fish once or twice a day, and only give them what they can eat in two to three minutes.

Different fish eat different foods. Flake food works for most small fish like tetras and guppies. Sinking pellets are better for bottom feeders like catfish and plecos. Live or frozen food like bloodworms and brine shrimp can be given as treats once or twice a week, but do not make them the main diet. If you have a mix of fish, you may need two or three types of food to make sure everyone gets fed.

Watch your fish eat. If food is hitting the bottom uneaten, you are feeding too much. If your fish are thin or seem to be fighting over food, you may be feeding too little. It takes a few weeks to find the right amount for your specific tank and fish.

Cleaning the filter without killing the bacteria

The filter is where the beneficial bacteria live—the bacteria that turn ammonia into nitrite and nitrite into nitrate, which is much less toxic. If you kill those bacteria by cleaning the filter wrong, your tank will cycle all over again and ammonia will spike.

Check your filter manufacturer's instructions for how often to clean it. Most filters need cleaning every two to four weeks, depending on how many fish you have. When you clean it, rinse the filter media in old tank water that you removed during your water change. Never use tap water, because chlorine kills bacteria. Gently squeeze the media in the old water until it looks cleaner, then put it back. If the filter has a mechanical pre-filter (a sponge or floss that catches big particles), you can rinse that more aggressively.

If your filter has a motor or impeller, do not submerge it. Wipe the outside with a damp cloth if it is dusty, but keep the electrical parts dry. Replace the filter media only when it falls apart or stops letting water through, not on a schedule.

Daily equipment checks that take two minutes

Every morning or evening, spend 60 seconds looking at your tank. Is the heater on? Is the filter running and making water move? Is the air pump bubbling (if you have one)? Are the fish acting normal, or are they gasping at the surface or hiding?

A heater failure can drop the temperature 10 degrees in a few hours, which stresses or kills tropical fish. A filter failure means ammonia starts building when ready. An air pump failure matters less in a tank with a filter that moves water, but it matters a lot in a tank with only an air stone. Catching these problems in the first hour makes a huge difference.

If a fish is gasping at the surface, the water is either too warm, too low in oxygen, or toxic. Check the temperature first. If it is normal, do a 50 percent water change when ready. If the fish still gasp after an hour, the problem is likely ammonia or nitrite, and you need to test the water and do another water change.

Cleaning the glass and decorations

Algae will grow on the glass, decorations, and plants. A small amount is normal and harmless, but too much blocks your view and can smother plants. Use an algae scraper (a plastic or magnetic tool made for aquariums) to clean the glass once a week. Scrape gently so you do not scratch acrylic tanks. Glass tanks are tougher and can handle a razor blade scraper if you need one.

For decorations and plants, you can gently rub them with your fingers during a water change, or use a soft brush. Do not use soap or chemicals. If algae is growing very fast, it usually means the tank is getting too much light or you are overfeeding. Move the tank away from a window, reduce the light to 8 to 10 hours a day, or feed less.

If you have live plants, trim dead leaves and stems so they do not rot in the water. Pinch off the top inch of stem plants once a month to make them bushier. Remove any plant that is turning brown or mushy, because it will decay and foul the water.

Seasonal tasks and when to replace things

Every few months, check the heater to make sure it is still accurate. If you have a thermometer, compare it to the heater's setting. A heater that is off by 5 degrees can stress your fish. Most heaters last two to five years before they fail or become unreliable.

Air pump tubing gets brittle and cracks after a year or two. If you notice the tubing is hard or cracked, replace it. Air stones clog with mineral deposits and algae; soak them in a bowl of old tank water and rub them gently, or replace them if they stop producing bubbles.

If you have a canister filter, check the seals and gaskets once a year. If they are cracked or dried out, water will leak. Most manufacturers sell replacement seal kits. If you have a hang-on-back filter, the intake tube can get clogged with algae; clean it with a small brush or pipe cleaner.

What to do when you go on vacation

Fish can go two weeks without food if the tank is established and stable. Before you leave, do a full water change and clean the filter. Feed your fish normally the day you leave. Do not use vacation feeding blocks—they dissolve unevenly and often foul the water.

If you are gone longer than two weeks, ask someone to do a 25 percent water change and feed a small pinch of food once during your absence. Write down exactly how much food that is (for example, "one pinch from the small spoon") so they do not overfeed. Leave your water test kit out with instructions on what to do if ammonia appears.

Before you leave, make sure the heater, filter, and air pump are all working. A power outage while you are away can kill fish in hours. If you live somewhere with frequent outages, consider a battery-powered air pump as backup.

Frequently Asked Questions

How often should I do a full water change instead of a partial one?

Never do a full water change on an established tank unless something has gone very wrong (like a chemical spill or a disease outbreak). A full change removes all the beneficial bacteria and restarts the nitrogen cycle. Stick to 25 percent partial changes weekly. The only exception is a brand-new tank during the first two weeks, when you may need to change water daily to keep ammonia down while bacteria are colonizing the filter.

Can I use bottled water or distilled water instead of tap water?

Bottled water is fine if it is not distilled. Distilled water has no minerals and will slowly leach minerals from your fish's bodies, stressing them. Tap water treated with dechlorinator is cheapest and works well. If your tap water is very hard or has high chlorine, a water conditioner designed for aquariums will help, but it is not always necessary.

What does it mean if my fish are hiding all the time?

Hiding usually means stress. Check the water temperature, test for ammonia and nitrite, and look for aggressive fish bullying others. If the water is fine, the tank may be too bright or too bare. Add plants or decorations to give fish places to feel safe. Some fish are naturally shy and hide more than others, so know what is normal for your species.

Do I need to do anything special if I add new fish?

New fish can introduce disease or ammonia spikes if you add too many at once. Add only a few fish at a time, and wait a week between additions so the filter bacteria can adjust. Quarantine new fish in a separate small tank for two weeks if possible, to make sure they are healthy before they go into the main tank. If you cannot quarantine, watch the main tank closely for signs of disease or ammonia problems.

How do I know if my filter is too small for my tank?

A filter that is too small will not keep up with waste, and ammonia or nitrite will stay elevated even after water changes. If your water test shows ammonia or nitrite above zero, and you are feeding normally and doing weekly water changes, the filter may be undersized. You can add a second filter, upgrade to a larger one, or reduce the number of fish. As a rule, the filter should turn over the entire tank volume at least three times per hour.