Start with a partial water change, not a full drain

The most common mistake with small aquariums is draining them completely and scrubbing everything. This strips away the bacteria that keep ammonia and nitrite from poisoning your fish. Instead, remove about 25 to 30 percent of the water, clean what you can see, and refill with fresh water at the same temperature.

Small tanks—typically 10 gallons or under—need cleaning more often than larger ones because waste builds up faster in less water. Plan to do a partial water change once a week. Between those changes, you can do a quick spot-clean of visible algae or uneaten food without removing fish.

Key Takeaways

  • Remove 25 to 30 percent of the water weekly rather than draining the tank completely, which kills the beneficial bacteria your fish depend on.
  • Use a gravel vacuum to siphon debris from the bottom while you drain water, which removes waste without disturbing the substrate too much.
  • Wipe the glass with a soft cloth or algae scraper, but never use soap, vinegar, or chemicals—only water or a dedicated aquarium cleaner.
  • Rinse the filter media in old aquarium water you just removed, not in tap water, to preserve the bacteria living in the filter.
  • Keep the temperature of replacement water within a few degrees of the tank water to avoid shocking your fish.

Use a gravel vacuum to clean the bottom without removing fish

A gravel vacuum is a tube with a wider chamber at one end. You place the chamber on the gravel, squeeze a bulb or siphon by mouth to start water flow, and debris gets pulled up into the tube while water drains into a bucket. This removes uneaten food and fish waste that settle on the bottom—the main source of ammonia buildup in small tanks.

Move the vacuum slowly across the gravel, especially in corners and under decorations where waste collects. You do not need to vacuum the entire bottom every week; focus on the dirtiest spots. If your tank has live plants, be gentler around the roots so you do not uproot them.

Clean the glass without chemicals

Algae on the glass is harmless to fish but makes the tank hard to see through. For light algae, use a soft cloth or a dedicated aquarium algae scraper—a plastic or rubber blade on a handle—and only water. Scrape gently so you do not scratch acrylic tanks (glass tanks are more forgiving).

Never use vinegar, soap, window cleaner, or any chemical. Even small amounts left behind can poison fish. If algae is heavy, a magnetic algae scraper lets you clean both sides of the glass at once by moving a magnet on the outside while the scraper blade moves on the inside.

Rinse the filter in old tank water, not tap water

The filter media—sponge, cartridge, or floss—traps waste but also hosts the bacteria that convert ammonia to less toxic compounds. If you rinse it in tap water, chlorine kills those bacteria and you lose the tank's biological balance. Instead, rinse the filter media in the water you just removed from the tank.

Squeeze the media gently in that old water until it runs clear. If the filter is very clogged and old water is not enough, use dechlorinated water or let tap water sit for 24 hours before using it. Replace the media only when it no longer cleans well even after rinsing—usually every 4 to 6 weeks, depending on the filter type and tank load.

Match the temperature of replacement water

Cold water added to a warm tank shocks fish and can trigger disease or stress. Before you add replacement water, check the tank temperature with a thermometer. Adjust the replacement water to within a few degrees of that temperature—ideally the same.

For small tanks, this is easiest if you let tap water sit in a bucket for a few hours to reach room temperature, or you can run it under warm (not hot) tap water while monitoring with a thermometer. Add the water slowly, pouring it gently so you do not disturb the gravel or stress the fish.

Clean decorations and plants only when needed

Rocks, driftwood, and plastic plants collect algae and waste, but they also host beneficial bacteria. Clean them only when algae is thick enough to block light or when you notice a smell. Use only water and a soft brush or cloth—no soap or chemicals.

If a decoration is very dirty, remove it, rinse it under tap water, and scrub with a soft brush. Live plants benefit from a gentle rinse but do not need scrubbing. If a plant is dying or rotting, remove it entirely rather than trying to clean it.

Watch for signs your tank needs more frequent cleaning

If the water turns cloudy within days of a water change, or if you smell ammonia (a sharp, urine-like odor), the tank is overloaded. This usually means too many fish, too much food, or a filter that is too small. Increase water changes to twice a week and check that you are not overfeeding—fish should eat all food within 2 to 3 minutes.

A properly maintained small tank should have clear water and no smell. If cloudiness or odor returns quickly even after cleaning, the problem is not cleaning frequency—it is tank capacity or feeding habits.

Frequently Asked Questions

How often should I do a full water change on a small aquarium?

Avoid full water changes. Stick to 25 to 30 percent partial changes weekly. A full drain removes the bacteria your fish depend on and can crash the tank. The only reason for a full change is if the water is poisoned or the tank has a serious disease outbreak—and even then, you should move fish to a temporary container first.

Can I use tap water straight from the faucet?

Tap water contains chlorine and sometimes chloramine, which kill fish and beneficial bacteria. Use a dechlorinator (a liquid or powder you add to the water) or let tap water sit uncovered for 24 hours so chlorine evaporates. Some aquarium filters have built-in carbon that removes chlorine, but dechlorinator is faster and more reliable.

What should I do if my fish hide during cleaning?

Hiding is normal—cleaning is stressful. Work quickly and gently, avoid sudden movements, and do not chase fish with the gravel vacuum. Keep the lights on so fish can see where they are going. If a fish stays hidden for more than an hour after cleaning, check that the water temperature is correct and that no sharp decorations are blocking escape routes.

Do I need to remove my fish before cleaning?

No. Removing fish stresses them and risks injury. Work around them during a partial water change and gravel vacuum. The only time to remove fish is if you are scrubbing a decoration heavily or if the tank is so small (under 5 gallons) that there is nowhere for them to hide during cleaning.

Why does my small tank get dirty so fast?

Small tanks have less water to dilute waste, so ammonia and nitrite build up quickly. The smaller the tank, the more often it needs cleaning. A 5-gallon tank may need a water change twice a week, while a 20-gallon tank might need one weekly. If your tank is constantly dirty, consider upgrading to a larger size or reducing the number of fish.