Start with water changes and testing

The single most important task in aquarium care is a partial water change — removing and replacing a portion of the tank water on a regular schedule. Most freshwater tanks need 25 to 30 percent of the water replaced weekly. Saltwater tanks often need 20 to 25 percent weekly because the salt buffer in the water breaks down faster.

Before you change the water, test it. A basic aquarium test kit measures ammonia, nitrite, nitrate, and pH. Ammonia and nitrite should always read zero; if either is present, increase water changes to twice weekly until they drop. Nitrate will climb over time — that is normal — but should stay below 40 parts per million. pH varies by fish species, so check what your fish need and keep it stable rather than chasing a perfect number.

Use a gravel vacuum (also called a siphon) to remove debris from the bottom while you drain the water. Uneaten food and fish waste settle there and break down into ammonia, which poisons the tank. The vacuum pulls it out before it becomes a problem. Fill the tank back with water that matches the temperature of the tank water — a difference of more than a few degrees stresses fish.

Key Takeaways

  • Change 25 to 30 percent of freshwater tank water weekly, and test ammonia, nitrite, and nitrate before each change to catch problems early.
  • Feed fish once or twice daily only what they can eat in two to three minutes, because uneaten food decays and poisons the water.
  • Clean the filter media monthly in old tank water, never tap water, because tap water kills the bacteria that break down fish waste.
  • Check heater and air pump function daily by watching for bubbles and temperature stability, since equipment failure can kill fish within hours.
  • Remove algae from glass weekly with a scraper or magnet, and trim live plants if they block light or crowd the tank.

Feed the right amount at the right time

Overfeeding is the most common mistake in aquarium keeping. Fish have small stomachs and do not need much food. A practical rule: feed only what the fish can eat in two to three minutes, once or twice daily depending on the species. Tropical fish usually eat once daily; goldfish and some bottom feeders do better with twice daily in smaller portions.

Uneaten food sinks and decays, creating ammonia that builds up in the water. Even a small amount of excess food can spike ammonia in a small tank within days. If you are unsure whether the fish are hungry, skip a feeding — fish can go weeks without food and will not starve from one missed meal.

Different fish eat different foods. Flake food works for most community fish. Sinking pellets work for bottom feeders like catfish and plecos. Herbivore pellets or algae wafers are needed for fish that eat plants. Check the species you own and match the food type; a fish that cannot eat the food you offer will go hungry while the food decays in the tank.

Maintain the filter and keep bacteria alive

The filter is where the bacteria live that convert fish waste into less toxic compounds. Never clean the filter media under tap water — chlorine in tap water kills the bacteria you need. Instead, rinse the filter media in a bucket of old tank water that you removed during a water change. Squeeze it gently until the water runs clear, then put it back.

Clean the filter media once a month, or more often if the flow slows noticeably. A clogged filter cannot process waste, and ammonia will spike. If the filter stops working entirely, bacteria die within hours and the tank becomes toxic. Check that water is flowing through the filter outlet every day — a quick glance is enough.

If you replace the filter cartridge, save the old one and run both together for a week before removing the old one. This lets bacteria colonize the new cartridge while the old one still works. If you replace the cartridge and remove the old one when ready, you lose most of your bacteria and ammonia will spike.

Monitor temperature and equipment daily

Fish are sensitive to temperature swings. Tropical fish need 76 to 80 degrees Fahrenheit; goldfish prefer 65 to 72 degrees. Check the thermometer every morning. If the temperature is off by more than a few degrees, the heater may have failed or the room temperature changed.

If you have an air pump (for an air stone or sponge filter), watch for bubbles every day. No bubbles means the pump has failed or the air line is blocked. Without aeration, oxygen levels drop and fish gasp at the surface. If the pump fails, replace it the same day.

Heater failure is more common than pump failure and harder to notice. A tank that is too cold slows fish metabolism and weakens their immune system; a tank that is too hot can kill fish in hours. A backup heater is cheap insurance in a tank with fish you care about.

Clean glass and manage algae growth

Algae grows on the glass and decorations when light and nutrients are present. A thin layer is harmless, but thick algae blocks the view and can shade live plants. Scrape the inside glass weekly with a plastic scraper or a magnetic algae cleaner. Never use metal or rough materials that scratch the glass.

Algae blooms happen when light runs too long or nutrients (especially nitrate) climb too high. If algae grows faster than you can scrape it, reduce light to 8 to 10 hours per day and increase water changes to twice weekly. Do not use algae-killing chemicals in a tank with fish — they can harm gills and the dead algae decays and spikes ammonia.

If you have live plants, trim them when they grow tall enough to block light from reaching plants below. Remove dead leaves and stems when ready — they decay like uneaten food. Healthy plants consume nitrate and slow algae growth, so they are worth the small effort to maintain.

Perform monthly deep checks

Once a month, do a more thorough inspection. Check all equipment for cracks or leaks. Look at the heater cord for damage. Inspect the filter housing for cracks. Test the thermometer against another thermometer to make sure it is reading correctly. If you have a power strip, make sure nothing is loose or wet.

Check the tank for signs of disease in the fish: torn fins, white spots, cloudy eyes, or fish that hide and do not eat. Early treatment is much easier than waiting until the fish is dying. A hospital tank (a small separate tank with a heater and filter) lets you treat sick fish without medicating the whole tank.

Look at the substrate (gravel or sand) for dead spots — areas where waste has accumulated and turned black. Vacuum these areas more thoroughly. If dead spots keep forming in the same place, water flow may be poor there and you may need to rearrange decorations to improve circulation.

Understand what changes mean trouble

Fish behavior is the fastest warning sign. If fish that normally swim actively are hiding, or if they gasp at the surface, something is wrong — usually ammonia, low oxygen, or temperature. Check the temperature first, then test the water. Do a large water change (50 percent) when ready if ammonia or nitrite is present.

Cloudy water usually means a bacterial bloom, which happens when ammonia spikes. Do not add chemicals; instead, do a 50 percent water change, reduce feeding to once every other day, and increase water changes to twice weekly until the water clears. The bloom will pass in a few days.

A strong smell from the tank means anaerobic bacteria (bacteria that live without oxygen) are growing in the substrate. Vacuum the gravel more thoroughly and increase water flow. If the smell persists, the tank may be overstocked or the filter may be undersized for the number of fish.

Frequently Asked Questions

How often should I do a complete tank cleaning?

Never do a complete water change and substrate cleaning at once — it kills the bacteria that keep the tank safe. Instead, do partial water changes weekly and vacuum the gravel during each change. If the tank is very dirty, do two water changes in one week instead of one, but always leave some old water and substrate bacteria intact.

Can I use tap water straight from the faucet?

Tap water contains chlorine and sometimes chloramine, which harm fish and kill filter bacteria. Let tap water sit in an open bucket for 24 hours before adding it to the tank, or use a dechlorinator product that removes chlorine when ready. Many aquarists keep a bucket of water aging for the next water change.

What should I do if I go on vacation for a week?

Do a full water change and clean the filter before you leave. Feed the fish normally the day before you go. Fish can survive a week without food — do not use automatic feeders, which often overfeed and foul the water. When you return, do a water change and resume normal feeding.

Why do my fish keep dying even though I change the water?

The most common causes are overfeeding, overstocking (too many fish for the tank size), or a filter that is too small. Check that the filter is rated for at least the volume of your tank. Count the fish and compare to stocking guidelines for your tank size. Reduce feeding to once daily and see if the fish survive longer.

Is a gravel vacuum really necessary?

Yes. Waste settles in the gravel and becomes ammonia. A gravel vacuum removes it before it poisons the tank. A siphon-style vacuum is cheap and works well. Without one, ammonia will build up and fish will get sick or die, even if you change the water regularly.