Clean your aquarium on a schedule, not when it looks dirty

The most reliable way to keep an aquarium clean is to do small maintenance tasks every week rather than waiting for the water to turn cloudy. A weekly 25 percent water change—removing about a quarter of the water and replacing it with fresh, dechlorinated water—removes waste that filters alone cannot catch. At the same time, you rinse the filter media (the sponge or cartridge inside the filter) under old aquarium water to clear trapped debris without killing the beneficial bacteria that live there.

The reason you use old aquarium water, not tap water, is that the bacteria you need live in that water and on the filter. Tap water will kill them. This is the single most important rule: never do a complete water change or rinse the filter under tap water unless the tank has crashed and you are starting over.

Monthly, you clean the glass and decorations. Quarterly, you do a deeper gravel vacuum in areas where waste collects. This rhythm keeps the tank stable and your fish healthy without the shock of sudden, drastic changes.

Key Takeaways

  • Change 25 percent of the water every week and rinse the filter media in old aquarium water to remove waste without harming beneficial bacteria.
  • Never use tap water to rinse the filter or do a complete water change unless the tank has crashed, because chlorine and chloramine kill the bacteria your fish depend on.
  • Clean the glass and decorations monthly with an algae scraper or soft cloth, and vacuum the gravel quarterly in high-waste areas.
  • Test the water for ammonia, nitrite, and nitrate every two weeks during the first month, then monthly after that, to catch problems before they harm your fish.
  • If the tank smells foul or the water turns cloudy overnight, do a 50 percent water change when ready and check for dead fish or uneaten food.

The weekly water change and filter rinse

Start by unplugging the filter and heater. Use a siphon (a tube that uses gravity to drain water) or a gravel vacuum (a tube with a wide head that sucks up debris from the bottom) to remove about 25 percent of the water. A gravel vacuum is better because it pulls waste from the substrate where it collects, but a plain siphon works if you do not have one yet.

While the water drains, turn off the lights and let the tank sit for a minute so debris settles. This makes the siphon more effective. Once you have removed the water, take the filter cartridge or sponge out and rinse it under a bucket of the water you just removed from the tank. Squeeze it gently until the water runs clear. Do not scrub it hard or use a brush—you are just clearing the visible gunk, not sterilizing it.

Refill the tank with fresh water that has been treated to remove chlorine and chloramine. Most tap water contains these chemicals to kill bacteria for human use, but they also kill the beneficial bacteria in your aquarium. Buy a dechlorinator at any aquarium store and follow the bottle's instructions—usually a few drops per gallon. Some dechlorinators also detoxify heavy metals. Let the new water reach the same temperature as the tank before adding it, or add it slowly so the temperature does not swing more than a few degrees.

Plug the filter and heater back in. The whole process takes 15 to 20 minutes once you have the supplies.

Cleaning the glass and decorations monthly

Algae grows 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 designed for aquariums—to gently scrape the inside glass. Avoid metal scrapers, which can scratch acrylic tanks. For the back and sides, a magnetic scraper lets you clean from outside without getting your hands wet.

Wipe decorations and plants with a soft cloth or old toothbrush. If algae is thick, soak the decoration in a bucket of tank water for 10 minutes, then scrub gently. Never use soap, bleach, or chemicals—they poison the water and your fish. If you have live plants, be gentle; some algae on leaves is fine and does not harm the plant.

If green algae (the cloudy kind that makes water look like pea soup) appears suddenly, it usually means too much light or too many nutrients. Reduce the light to 8 to 10 hours per day and do a 50 percent water change. Do not add fertilizer until the bloom clears.

Vacuuming the gravel quarterly and spot-cleaning weekly

Fish waste and uneaten food sink to the bottom and create pockets of ammonia-rich debris. A gravel vacuum removes this without disturbing the substrate too much. Once a month during your regular water change, spend an extra few minutes vacuuming the corners and under decorations where waste collects most.

Every week, do a quick spot-clean: look for any visible uneaten food or dead plant matter and remove it by hand or with tweezers. Dead food rots and spikes ammonia levels fast. If you see a fish that has not moved in several hours, it may be dead—remove it when ready.

Quarterly, do a deeper vacuum of the entire substrate. This is more disruptive, so do it during a larger water change (50 percent instead of 25 percent) so the fish have less stress. You will stir up a lot of debris, which is normal. The filter will clear the water over the next few hours.

Testing water quality to catch problems early

A water test kit measures ammonia, nitrite, and nitrate—the three waste products that build up in a tank. You can buy a liquid test kit (more accurate) or test strips (faster but less precise). Test every two weeks for the first month after setting up the tank, then monthly after that.

Ammonia and nitrite should always be zero. If either is above zero, do a 50 percent water change when ready and check for dead fish, uneaten food, or an overstocked tank. Nitrate will slowly climb over time and is less urgent, but if it reaches 40 parts per million or higher, do a larger water change (50 percent) to bring it down. Nitrate is not toxic at moderate levels, but high levels stress fish and promote algae.

If you see ammonia or nitrite spike after months of zero readings, the filter may have failed or the tank may be overstocked. Do a large water change and investigate what changed—did you add too many fish, stop doing water changes, or overload the filter with decorations?

What to do if the water turns cloudy or smells bad

Cloudy water overnight usually means a bacterial bloom—too many bacteria growing at once because something has died or decayed. Check for a dead fish, dead plant, or a large pile of uneaten food. Remove it. Then do a 50 percent water change and increase aeration by turning up the filter flow or adding an air stone.

A foul smell means anaerobic bacteria (bacteria that live without oxygen) are breaking down waste in dead zones under the gravel. This happens when the substrate is too deep, the filter is too weak, or the tank is overstocked. Do a 50 percent water change, vacuum the gravel thoroughly, and check that the filter is running at full strength. If the smell returns, the tank may need a larger filter or fewer fish.

Brown water that smells like tea is usually tannins from driftwood or peat, which is harmless but looks bad. It clears with water changes and activated carbon in the filter, but it is not urgent.

Supplies you need to get your free guide

You do not need much to keep an aquarium clean. A gravel vacuum or siphon costs $15 to $30 and will last years. A dechlorinator bottle costs $5 to $10 and treats hundreds of gallons. An algae scraper costs $3 to $8. A water test kit costs $20 to $35 for a liquid kit or $5 to $10 for test strips. A soft cloth or old toothbrush you probably already have.

Buy these items before you set up the tank so you are ready for your first water change. Aquarium stores and online retailers carry all of them. Do not skip the dechlorinator—it is the most important item on this list and the easiest to forget.

Frequently Asked Questions

Can I use bottled water instead of tap water?

Bottled water works if it is not distilled. Distilled water has no minerals and will throw off the tank's chemistry. Spring water or purified water is fine, but it costs more than treating tap water with dechlorinator. If your tap water is very hard or has high chloramine levels, bottled water may be worth it—ask your local aquarium store about your water.

How often should I replace the filter cartridge?

Never throw away a filter cartridge unless it falls apart. Rinse it in old tank water every week and it will last months or years. The beneficial bacteria live on the cartridge, so replacing it kills them and crashes the tank. Only replace it if it tears or clogs so badly that water barely flows through.

Is it okay to do a big water change instead of small weekly ones?

No. A 50 percent change once a month stresses fish more than four 25 percent changes spread across the month. Small, frequent changes keep the water stable and the fish calm. If you miss a week, do a 50 percent change the next week to catch up, but do not make it routine.

What if I go on vacation for two weeks?

Do a 50 percent water change and clean the filter the day before you leave. Feed the fish lightly the day you go—uneaten food will foul the water faster than hunger will harm the fish. Most fish can go two weeks without food. If you are gone longer, ask someone to do a 25 percent water change and feed lightly once during your absence.

Do I need to clean the substrate if I have a strong filter?

A strong filter clears the water column but cannot reach waste buried in the gravel. Vacuuming removes ammonia that would otherwise spike. Even with the best filter, you need to vacuum at least quarterly. Weekly spot-cleaning of visible debris is even better.