How to Clean a Fish Tank: Water Changes, Gravel Vacuums and Algae

Clean a fish tank by changing part of the water with a gravel vacuum, not by emptying it. The RSPCA suggests 30 percent every week; RSPCA Australia 10 to 25 percent weekly; VCA every 2 to 4 weeks. Treat new water with conditioner, match its temperature, rinse filter media in old tank water, and never use soap.

How much water, and how often?

The sources disagree, so here they are together:

The veterinary and welfare sources lean toward small weekly changes; the Marina manual’s cap of 20 percent is the most cautious. A weekly change in the 10 to 25 percent range satisfies most of the table. Heavy waste producers such as goldfish may need the top of the range; see the goldfish chapter.

Step by step

1. Unplug the heater and filter. The Marina manual says to disconnect every appliance before putting your hands in the water. Aqueon says a heater must be unplugged and cooled for at least 30 minutes before it comes out of the water.
2. Scrape the glass. Use an algae scraper or magnetic cleaner before the vacuum, so loosened algae gets siphoned out.
3. Vacuum the gravel. Push the gravel vacuum into the substrate and let the flow lift waste out while the gravel falls back. Work across part of the bottom each time.
4. Stop at your target amount. Mark the bucket so you know when you have removed 10 to 25 percent.
5. Rinse filter media in the old water, if due (see below).
6. Refill with treated water at a similar temperature to the tank, as the Marina manual says, dosed with water conditioner.
7. Plug everything back in once the heater has been back in the water for 30 minutes, per Aqueon, with drip loops in the cords.

Gravel vacuums, siphons and algae scrapers

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Filter care without losing the cycle

The helpful bacteria live mainly in the filter. RSPCA Australia says to rinse filter media in tank water, never tap water, monthly or when clogged. The Marina manual replaces cartridges every 2 to 4 weeks but never all at once, to keep the biological filtration going. The filters chapter covers each filter type, and the cycling chapter explains why the bacteria matter.

Algae

Some algae is normal. The usual causes of too much are light and leftover food. The RSPCA suggests 8 to 10 hours of light a day and VCA says keep the tank out of direct sunlight; a timer helps, as in aquarium lighting. Feeding less also means less waste in the water; see fish food by fish.

For the glass, a plastic or blade scraper works on algae spots, and a magnetic cleaner lets you wipe the inside of the glass from the outside. Check any blade scraper is safe for your tank material, since acrylic scratches.

Daily and monthly checks

The Marina manual asks for a quick daily look: remove visible debris or leftover food and check the filter is running smoothly. Check the thermometer at the same time. Once the tank is established, RSPCA Australia says testing the water every one to two weeks is enough.

Water conditioners and test kits for water changes

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If fish look unwell after a clean, test ammonia and nitrite first, and see a veterinarian who treats fish if they do not improve. The setup checklist lists the rest of the gear.