How to clean a fish tank without harming the fish

A fish tank looks simple, but it is a small ecosystem. Fish produce waste. The waste breaks down into ammonia, and bacteria living in the filter, the gravel and the decorations turn that ammonia into less harmful compounds. Cleaning is about removing the waste while keeping those bacteria alive. A tank cleaned properly is stable and healthy. A tank cleaned with soap, scalding water or a full water change can lose its fish within hours.

This guide covers the routine a tank needs each week and each month, the products to avoid, and the warning signs that a tank needs attention now. It is written for freshwater tanks, which are the most common at home. Saltwater and reef tanks need extra care that this guide does not cover.

Before you start

Unplug the heater and the pumps before you remove any water or touch the equipment, so nothing runs dry or burns out. Turn off the lights, and move slowly so the fish stay calm. Use a bucket that is used only for the tank and kept free of soap and cleaners. Use water at room temperature, close to the temperature of the tank, to avoid stressing the fish.

Keep a notebook or a note on your phone with the date of each water change, the test results, and any changes to the fish or the plants. A record shows trends that a single test hides.

The weekly partial water change

A partial water change is the most important routine task. Remove about 10 to 25 percent of the water with a gravel vacuum. The vacuum lifts waste from the gravel as it draws out the water. Treat the new water with a conditioner that removes chlorine and chloramine, match its temperature to the tank, and add it slowly. Test the water before and after, because the right amount of change depends on the number of fish and the plants.

Do not change all of the water at once. A large, sudden change can shock fish, upset the bacteria, and cause temperature and chemistry shifts that stress the fish. A regular partial change is far safer than an occasional big one.

Never refill straight from a hot tap. Water that is too hot or too cold can shock the fish. Water softened by a salt-based system can carry too much sodium for some fish, so test the tap water first if you are unsure.

How the calculator works

The water change calculator works out how much water to take out of your tank. Enter the tank’s length, width and height in inches and the percentage you want to change. It gives the tank’s volume in US gallons, the gallons to change, and the liters in brackets.

The volume of a rectangular tank is length x width x height in cubic inches. Divide by 231, the number of cubic inches in a US gallon, to get gallons. The water to change is gallons x percent / 100. With the default 24 x 12 x 16 inch tank, that is 4608 cubic inches, or 19.9 gallons, and 20 percent of it is about 4 gallons, which is 15.1 liters.

The calculator accepts 10 to 25 percent, the range the guide gives, for a reason that comes from the tank itself. Bacteria in the filter, the gravel and the decorations turn the ammonia from fish waste into less harmful compounds. A large, sudden change can shock the fish, upset those bacteria, and shift the temperature and the water chemistry. A regular partial change removes waste and leaves the bacteria in place, which is far safer than an occasional big one. A heavily stocked or new tank may need changes more often, so test the water and follow the advice for your fish.

The answer treats the tank as a full glass box. Gravel and decor take up room, so the real volume of water is lower and the figure is slightly high. Use it to size the bucket, not as a measurement. Match the temperature of the new water to the tank, and treat it with a conditioner before it goes in.

Vacuuming the gravel

Push the gravel vacuum into the substrate, but not all the way to the bottom, so it lifts waste without sucking up the whole bed. Work across the tank in sections. Leave some of the gravel undisturbed in each session, because bacteria live in it as well as in the filter. Rinse the vacuum in the bucket of tank water between sections, not under the tap.

Sand needs a gentler approach. Wipe the surface lightly with the tip of the vacuum rather than digging into it. That keeps the sand from being sucked up and leaves the bacteria undisturbed.

Cleaning the glass

Algae builds up on the glass, and a film makes the tank look dull. Use a magnetic algae scraper or a clean sponge kept for the tank only. Clean the inside walls while the tank is partly drained, so the scrapings are removed with the water. Do not use cloths or scouring pads that have been used for household cleaning, because they carry residue that can harm fish.

Acrylic tanks scratch more easily than glass. Use a soft pad made for acrylic, and never a scraper with grit on it. A scratched wall collects more algae, so careful cleaning saves work later.

Cleaning the filter

Filters are the home of the bacteria that keep the water safe. Clean them only when the flow drops, rather than on a fixed schedule that disturbs the bacteria too often. When you do clean, rinse the media in a bucket of water taken from the tank during a water change. Squeeze sponges gently until the dirt comes out, and do not wash them in tap water. Chlorine and chloramine kill the bacteria, and a filter that loses them can make the water fail within days.

Replace cartridges and carbon according to the manufacturer’s guide. Carbon stops working once it is saturated, and an old carbon cartridge can release what it has absorbed. Stagger the cleaning, so that half the filter is cleaned one week and the other half the next. That keeps a colony of bacteria in the tank at all times.

Decorations and plants

Ornaments can be cleaned in water taken from the tank. Rinse them in warm tap water if they are very dirty, then let them dry before they go back in the tank. Plastic plants can be scrubbed with a soft brush in tank water, which removes most of the film.

Live plants need trimming and checks for dead leaves, which rot and foul the water. Remove any leaf that has gone brown or slimy at once. Do not clean live plants with soap, and do not scrub the leaves hard, because the leaves are fragile and the bacteria also live on their surfaces.

Products to avoid

• Soap, detergent, dish soap and any household cleaner, which leave residue that can kill fish, even after rinsing
• Bleach, which can kill fish and bacteria
• Scented cleaners and air fresheners near the tank, which can drift onto the water
• Tap water to rinse filter media, because chlorine and chloramine kill the bacteria
• Scouring pads and scrapers with grit on them, which scratch glass and acrylic
• Medicines used without a clear reason, which can upset the balance of the tank and harm the filter bacteria

Signs the tank needs attention now

Watch for fish that gasp at the surface, hang near the filter or heater, rub against the gravel or the decorations, or show red streaks, clamped fins or white spots. A cloudy tank, a bad smell, dead fish, or a sudden change in the water color all mean the tank needs testing and a partial water change.

Test the water for ammonia, nitrite and nitrate, and act on the results. In a mature, cycled tank, ammonia and nitrite should read zero, and nitrate should stay low. Any reading above zero for ammonia or nitrite means the water needs a change and a closer look at the cause, such as overfeeding or a dead fish hidden in the decorations.

Feeding and stability

Feed the fish only what they eat in two or three minutes, once or twice a day. Excess food rots and adds ammonia to the water. Keep the tank out of direct sun, which feeds algae, and away from drafty windows and doors, which chill the water. Do not overstock the tank. More fish means more waste, and more waste means more cleaning and more stress.

If the fish jump, keep a lid on the tank with gaps small enough to stop them escaping. Fish that jump out rarely survive.

A monthly check

Once a month, check the equipment: the heater, the pump, the impeller, the air pump and the lid. Clean any build-up from the heater with a cloth and water, with the heater unplugged and cooled. Replace worn tubing, and check the seals on the hood and the lights. Read through the test log, and see whether the readings have been steady or drifting.

Keeping children and pets safe

Keep the tank out of reach of pets, and supervise children who want to put their hands in the water. Electrical equipment and water need respect. Plug the tank into a ground-fault circuit interrupter (GFCI) outlet if you can, and keep cables clear of the water and away from wet hands. Do not place the tank where it can be knocked over.

When to call a specialist

Call a vet who treats fish, or an aquarium specialist, if fish are dying without a clear cause, if disease spreads to healthy fish, or if the water cannot be stabilized after several changes. A specialist can test the water and the fish, and suggest a treatment that suits the species. Bring your test results and a description of your routine, because that information helps them find the cause faster.