The basics: water changes, filtration, and feeding

An aquarium stays healthy when you do three things regularly: change some of the water, keep the filter running, and feed your fish the right amount. These three tasks prevent the buildup of waste that poisons fish and kills beneficial bacteria. Most aquariums need a partial water change once a week — usually 25 to 30 percent of the tank volume — and a working filter that runs 24 hours a day.

Feeding is where most new aquarium keepers make their first mistake. Fish need less food than you think. A good rule is to feed only what they can eat in two to three minutes, once or twice a day depending on the species. Uneaten food rots and fouls the water. Overfeeding also makes fish produce more waste, which raises ammonia and nitrite levels that stress or kill them.

The filter is your second line of defense. It traps solid waste and grows colonies of bacteria that convert ammonia (from fish waste) into nitrite, then into nitrate, which is less toxic. Never replace the filter media with completely new material all at once — that kills the bacteria you need. Instead, rinse the old media in water you removed from the tank during your water change, then put it back. Replace only a portion of it every few months.

Key Takeaways

  • Change 25 to 30 percent of the water once a week to remove waste and keep chemical levels stable.
  • Feed only what your fish can eat in two to three minutes, once or twice daily, because uneaten food and overfeeding both foul the water.
  • Run your filter 24 hours a day and rinse the media in old tank water during water changes, not tap water, to preserve the bacteria that keep the tank safe.
  • Test ammonia, nitrite, and nitrate levels with an inexpensive test kit when you first set up the tank and again if fish start dying or acting sick.
  • Remove dead fish, uneaten food, and visible algae buildup as soon as you notice them to prevent water quality problems.

Setting up the nitrogen cycle before adding fish

A new aquarium needs time to build up the bacteria that make it safe for fish. This process is called cycling, and it takes two to four weeks. During this time, ammonia from decaying food or fish waste is converted by bacteria into nitrite, then into nitrate. If you add fish before this cycle is complete, ammonia and nitrite will spike and kill them.

You can cycle a tank three ways. The oldest method is to add a small amount of food or a dead shrimp and wait for bacteria to colonize the filter — this takes the full four weeks and smells bad. A faster method is to add a commercial bacterial starter (sold as "beneficial bacteria" or "tank starter") which introduces the bacteria directly. The fastest method is to seed your filter with media from an established tank, which transfers bacteria when ready.

Test the water every few days with an ammonia and nitrite test kit. When both read zero for three days in a row, the cycle is complete and the tank is ready for fish. If you add fish before this happens, do it slowly — add only a few hardy fish first, then wait a week before adding more. This gives the bacteria time to handle the extra waste.

Cleaning the tank without harming the bacteria

The goal of cleaning is to remove waste without destroying the filter bacteria that keep the tank safe. During your weekly water change, use a gravel vacuum (a tube with a bulb or electric pump) to suck debris from the bottom of the tank while you drain water. This removes solid waste before it breaks down and fouls the water. Gravel vacuums are inexpensive and available at any pet store.

Wipe algae off the glass with a soft cloth or an algae scraper — never use soap or chemicals. If algae is thick on decorations or plants, remove them and scrub them under running tap water, then put them back. Avoid scrubbing the filter itself; instead, rinse the filter media gently in a bucket of water you removed from the tank. Tap water contains chlorine that kills bacteria, so always use old tank water for rinsing.

Never do a complete water change unless the tank has a serious problem — a leak, a chemical spill, or a disease outbreak. A full drain removes all the bacteria and restarts the cycle. If the water looks cloudy or smells bad, do a larger partial change (50 percent) and check your feeding and filter for problems.

Testing water and recognizing problems early

An inexpensive aquarium test kit tells you the levels of ammonia, nitrite, nitrate, and pH. You need one when you first set up the tank to confirm the cycle is complete, and again if fish start acting sick or dying. Kits come as liquid drops (more accurate) or test strips (faster but less reliable). A basic liquid kit costs $20 to $30 and lasts for hundreds of tests.

Safe levels are: ammonia 0 ppm, nitrite 0 ppm, nitrate below 40 ppm, and pH between 6.5 and 7.5 for most freshwater fish. If ammonia or nitrite are above zero, you are overfeeding or the filter is not working — reduce food and check that the filter is plugged in and water is flowing through it. If nitrate is above 40 ppm, you are not changing water often enough; increase to twice-weekly changes.

Fish that gasp at the surface, hide constantly, or have torn fins are usually stressed by poor water quality. A cloudy tank or a strong smell also signals a problem. Test the water first, then fix the cause — usually overfeeding, a clogged filter, or skipped water changes — rather than adding chemicals or medications.

Maintaining the filter and air pump

The filter is the engine of your tank. Check it weekly to make sure water is flowing through it at a steady rate. If flow slows, the media is clogged and needs rinsing. Rinse it in old tank water (never tap water) by gently squeezing the media in a bucket of water you removed during your water change. This removes trapped waste without killing the bacteria.

If your tank has an air pump (which creates bubbles and helps oxygen dissolve into the water), check the air stone or diffuser monthly. If bubbles slow or stop, the stone is clogged. Soak it in hot tap water for 10 minutes to dissolve mineral buildup, then rinse and put it back. Replace the air pump tubing if it cracks or becomes cloudy, as this reduces air flow.

Most filters and air pumps last two to five years with regular cleaning. If a filter stops working and cleaning does not fix it, you will need to replace it. When you do, save the old filter media and put it in the new filter to transfer the bacteria and avoid restarting the cycle.

Dealing with algae and unwanted growth

Algae grows when there is too much light and too many nutrients in the water. Green water (algae floating freely) or green slime on the glass is normal in a healthy tank, but thick brown or black algae means something is wrong. Usually it is overfeeding, too much light, or infrequent water changes.

To control algae, reduce light to 8 to 10 hours a day, increase water changes to twice weekly, and cut back on feeding. Wipe the glass and gravel vacuum the bottom during each change. Do not use algae-killing chemicals — they kill the algae but leave dead matter that fouls the water. Manual removal and fixing the root cause work better.

Some algae-eating fish like plecos or otocinclus catfish help, but they are not a substitute for maintenance. They produce waste like any other fish and need proper care themselves. If you add them, do not overfeed thinking they will eat the extra — they will not, and the food will still foul the water.

Seasonal maintenance and equipment checks

Every three months, inspect the heater (if you have one) to make sure it is maintaining the right temperature. Most tropical fish need 76 to 78 degrees Fahrenheit. If the water feels cold, the heater may be failing. Test it by unplugging it for a few hours and checking that the temperature drops, then plugging it back in and checking that it rises. If it does not, replace it.

Check all hoses and connections for cracks or leaks. A small leak can become a big one, and a failed hose can drain your tank. Replace any tubing that is cloudy, cracked, or brittle. Check the power cord on the filter and air pump for damage, and make sure they are plugged into a outlet that is not controlled by a wall switch — if someone turns off the switch, your filter stops and the cycle breaks.

If you have live plants, trim dead leaves and stems weekly so they do not rot and foul the water. If plants are growing too fast and shading the rest of the tank, thin them out. Dead plant matter breaks down like uneaten food and raises ammonia levels.

When to call for help or replace equipment

Most aquarium problems are caused by overfeeding, infrequent water changes, or a clogged filter — all things you can fix yourself. But some problems need new equipment. If the filter stops working after cleaning and rinsing, it is time to replace it. If the heater does not maintain temperature, replace it. If the air pump is silent and soaking the air stone does not restore bubbles, the pump itself is failing.

If fish are dying despite correct water parameters and regular maintenance, the problem may be a disease or a fish that is not compatible with your tank conditions. Research the species you have to confirm their temperature and water chemistry needs. Some fish need cooler water, higher pH, or more space than others. If you have the wrong fish for your tank, moving them to a more suitable home is kinder than trying to force them to adapt.

Never use medications or chemicals without testing the water first and confirming what the problem is. Many aquarium chemicals can harm fish if used wrong, and some kill the beneficial bacteria you need.

Frequently Asked Questions

How often should I change the water in my aquarium?

Most tanks need a 25 to 30 percent water change once a week. If you have more fish than the tank is designed for, or if you overfeed, change water twice a week. If the tank is lightly stocked and you feed carefully, you may be able to go longer, but weekly is the safest routine.

Can I use tap water in my aquarium?

Tap water is usually safe for aquariums, but it contains chlorine that can harm fish and kill beneficial bacteria. Let tap water sit in an open bucket for 24 hours before adding it to the tank, or use a dechlorinator product (a few drops per gallon) to remove chlorine when ready. Always use old tank water for rinsing filter media.

Why are my fish gasping at the surface?

Fish gasp when oxygen levels are low or water quality is poor. Check ammonia and nitrite with a test kit — if either is above zero, do a large water change and reduce feeding. Make sure the filter and air pump are running. If the water is warm (above 80 degrees), cool it down by turning off the heater or doing a water change with cooler water, as warm water holds less oxygen.

How long does it take to cycle a new aquarium?

Cycling takes two to four weeks if you add food or a dead shrimp and wait for bacteria to grow naturally. Using a bacterial starter product cuts this to one to two weeks. Seeding the filter with media from an established tank can cycle a tank in days. Test ammonia and nitrite every few days to know when it is safe to add fish.

What should I do if my filter stops working?

First, check that it is plugged in and the power switch is on. If it is, clean or rinse the filter media — a clogged filter often looks like it is broken. If cleaning does not restore flow, the motor may have failed and you will need to replace the filter. When you do, save the old media and put it in the new filter to keep the bacteria alive.