Start with a partial water change, not a full drain

The fastest way to ruin a fish tank is to empty it completely and scrub it clean. Your aquarium's beneficial bacteria live in the gravel, filter, and tank walls — removing all the water kills most of them, and your fish will suffer ammonia spikes that can be fatal. Instead, remove about 25 percent of the water, clean what you can reach without disturbing the gravel, and refill with fresh water at the same temperature.

This partial-change method works because the remaining water and surfaces keep the bacterial colony alive while you remove waste and refresh the tank. Most home aquariums need this kind of cleaning once a week if you have a filter, or twice a week if you do not. The exact timing depends on how many fish you have and how much they eat — more fish means more waste and faster water degradation.

Key Takeaways

  • Remove about 25 percent of the water each cleaning session, never drain the tank completely unless the fish are sick or the tank is broken.
  • Clean the glass, decorations, and filter intake during your water change, but leave the gravel mostly undisturbed to preserve beneficial bacteria.
  • Use only a siphon or aquarium vacuum to remove debris from the gravel without stirring up the whole layer.
  • Refill with fresh water that matches the tank temperature, and wait a few minutes before turning the filter back on if you removed it.
  • A full tank scrub with gravel removal should happen only once or twice a year, or when algae buildup or fish illness makes it necessary.

Gather your cleaning supplies before you start

You need a bucket (one you use only for the aquarium, never for household chemicals), a siphon or aquarium vacuum, an algae scraper or soft cloth, and a thermometer. The bucket should hold at least 5 gallons if you have a small tank, or 10 gallons for a larger one. Do not use soap, bleach, or any cleaner — plain water is all you need, because chemicals will poison your fish even in tiny amounts.

An aquarium vacuum is a tube with a bulb at the top that sucks gravel debris into a bucket without removing the gravel itself. A basic siphon (a long tube you start by mouth suction) works just as well and costs less. An algae scraper is a plastic or rubber blade on a handle — avoid metal scrapers, which can scratch acrylic tanks or damage the glass coating. If you do not have a scraper, a soft cloth or old toothbrush removes algae just as well, though it takes longer.

Remove the water and clean the glass

Place your bucket next to the tank. If you are using a siphon, fill the tube with water from the tank, pinch both ends to trap the water inside, place one end in the bucket (lower than the tank), and release. Water will flow out until the bucket is full or the tank is at your target level. If you are using an aquarium vacuum, push the tube down into the gravel, squeeze the bulb at the top to start suction, and hold the intake end steady as debris flows into the bucket.

As the water level drops, use your algae scraper or cloth to wipe the inside of the glass, working from top to bottom. Algae scrapes off easily when the water level is lower and you have room to move. Do not scrape the back wall if you want to leave some algae there — it provides food for some fish and helps balance the tank. Wipe the front and sides where you watch the fish, and leave the back and corners alone unless algae is very thick.

Clean the filter intake and decorations

While the water is draining, look at your filter intake — the tube or opening where water enters the filter. If it is clogged with algae or debris, rinse it gently under the tap water you are about to add back to the tank. Do not remove the filter cartridge or media unless it is visibly clogged; rinsing the intake is usually enough. If the cartridge is very dirty, rinse it in a bucket of old tank water (not tap water), because tap water kills the bacteria living on the cartridge.

Wipe down any decorations, plants, or rocks with a soft cloth or old toothbrush. Debris and algae collect on these surfaces, and removing them keeps the tank cleaner longer. If a decoration is very dirty, remove it and scrub it under running water, then return it to the tank. Artificial plants can be rinsed; live plants should be left mostly alone unless they are visibly decaying.

Refill the tank with fresh water at the right temperature

Before you add new water, check the temperature of the water still in the tank using your thermometer. Most tropical fish need water between 76 and 80 degrees Fahrenheit; goldfish and other cold-water fish prefer 65 to 72 degrees. Fill your bucket with tap water and let it sit for a few minutes, then check its temperature. If it is too cold, add a little hot water from the tap until it matches the tank temperature. If it is too hot, let it cool or add cold water.

Pour the water slowly into the tank, aiming for a corner or against a decoration so the flow does not stir up the gravel or stress the fish. Fill until the tank reaches its normal level — usually marked on the side or about an inch below the rim. Turn the filter back on once the water level is stable. If you removed the filter intake to clean it, wait 30 seconds before turning the filter on so air does not get trapped in the lines.

Do a full tank cleaning only once or twice a year

A complete cleaning — where you remove all the water, take out the gravel, and scrub every surface — should happen only when the tank is heavily soiled, when you are treating a fish disease, or when you are moving the tank. This process kills most of the beneficial bacteria, so your tank will need a week or two to recover before the water chemistry stabilizes again.

If you do need a full cleaning, remove the fish to a temporary container with some of the old tank water, drain the tank completely, remove the gravel into a bucket, and rinse the gravel under running water until the water runs clear. Scrub the tank with a cloth and plain water, rinse it thoroughly, return the gravel, refill with fresh water at the right temperature, and return the fish. Do not add new gravel or decorations during this process unless the old ones are broken or diseased.

Watch for signs your tank needs cleaning more often

If the water looks cloudy or smells bad, the tank needs cleaning sooner than your normal schedule. Cloudiness usually means a bacterial bloom — harmless to fish but a sign that waste is building up faster than the filter can handle. A bad smell means ammonia or hydrogen sulfide is accumulating, which is dangerous. Do a partial water change when ready, and increase your cleaning frequency to twice a week until the water clears.

If algae is growing so thick that you cannot see the fish, or if the gravel looks dark and sludgy, the tank is overloaded. This usually happens when you have too many fish for the tank size, or when you are feeding too much. Increase water changes to twice a week, reduce feeding by half, and consider moving some fish to a larger tank or a separate container. A tank that is properly stocked and fed should never need more than one cleaning per week.

Frequently Asked Questions

Can I use tap water straight from the faucet?

Yes, but let it sit for a few minutes so the temperature matches the tank. If your tap water is heavily chlorinated, you can let it sit for 24 hours before adding it, or buy a dechlorinator from a pet store. Most modern tap water is safe for fish after a few minutes of aeration, but if you notice fish gasping at the surface after a water change, your tap water may need treatment.

What if I accidentally drain the whole tank?

Do not panic. Refill it when ready with fresh water at the right temperature, and do not add any new fish for at least a week. The tank will go through a cycle where ammonia spikes, then nitrite spikes, then both drop as bacteria rebuild. Test the water every few days if you have a test kit, or watch the fish for signs of stress like gasping or hiding. Do partial water changes every other day for the first week to keep ammonia levels down.

How often should I clean the filter?

Check the filter intake weekly during your water change, and rinse it if it is clogged. The filter cartridge or media should be rinsed in old tank water once a month, or more often if the flow slows down. Never replace the cartridge with a brand-new one unless the old one is falling apart — reuse it as long as it holds water, because it contains the bacteria your tank needs.

Is it okay to use a gravel vacuum if I have live plants?

Yes, but be careful around the plant roots. Vacuum the open gravel between plants, and avoid sucking up the roots or pulling plants out of the substrate. If a plant is rooted in the gravel, use a soft cloth to clean around it instead of the vacuum. Rooted plants actually help keep the gravel clean by absorbing waste, so they reduce how often you need to vacuum.

What should I do if the water turns brown or yellow?

Brown or yellow water usually means tannins from decorations or driftwood, which are harmless but look bad. Do a partial water change and add activated carbon to your filter if you have room. If the water is cloudy and brown, it is likely a bacterial bloom from overfeeding or too many fish — do a water change, reduce feeding, and increase cleaning frequency. If the water smells like rotten eggs, there is a serious ammonia problem and you need to do a large water change when ready.