How often should you change aquarium water?

Why aquarium water needs changing at all

Fish excrete ammonia all the time. In a cycled tank, bacteria in the filter turn it into nitrite and then into nitrate (we explain the process in Cycling an aquarium: the nitrogen cycle). Nitrate is the last link in that chain, and the filter does not remove it. According to the MSD Veterinary Manual, nitrate leaves the system only through water changes, anaerobic bacterial activity or uptake by live plants. In a typical home tank, plants and anaerobic zones are not enough, so water changes remain the main tool.

Changing water is not the same as topping up. When water evaporates, everything dissolved in it stays behind. MSD warns that if old water is never removed, organic acids produced by fish and bacteria build up. They slowly push the pH down until the biological filter slows. The result is so-called old tank syndrome: the water looks clear, yet the fish keep getting worse.

How much water, and how often

There is no single number, and the sources admit it. The RSPCA aquarium guidance calls for weekly 30% water changes, yet its step-by-step method mentions siphoning out ten percent. MSD lists a water change every 2 weeks in its routine-care schedule and adds that the amount depends on stocking level. The difference is not an error: a tank with a few neon tetras and a tank full of larger fish do not produce the same amount of waste.

For most beginners, a quarter to a third of the water each week is a sensible starting point, but let your test results have the final say. OATA, the UK ornamental fish trade association, sets a maximum of 50 mg/L of nitrate above your tap water level for tropical freshwater tanks. If the test shows nitrate creeping towards that limit between changes, change water more often or in larger amounts. If it stays low, your routine is working.

Unfiltered containers, such as bowls and small betta tanks, need more. MSD mentions up to 50% of the water per week for them. We explain why a bowl is the wrong home in our goldfish care guide, and the Siamese fighting fish also needs a filtered, heated tank.

Why not change all the water at once

Fish are adapted to the water they live in. OATA therefore asks that new water be similar in hardness, pH, temperature and dissolved oxygen to the water the fish are kept in. The RSPCA adds that new water at a different temperature will shock the fish. Replace everything, and all of those values shift at the same moment.

In an old, neglected tank, a big change can even be dangerous. MSD explains that a sudden rise in pH can make the ammonia already present toxic. In a tank like that, test first, and make larger changes only with care, ideally with professional advice.

Tap water: chlorine, chloramine and local differences

MSD reports adverse effects in fish at chlorine levels as low as 0.02 mg/L and deaths at 0.04 mg/L. Some water suppliers use chloramine, a compound of chlorine and ammonia. According to the University of Florida (UF/IFAS), chloramine does not gas off as easily as chlorine alone, so leaving a bucket to stand overnight is not enough. The reliable answer is a water conditioner that binds both chlorine and chloramine.

What comes out of your tap depends on your supplier, and it varies even within one region. In Ljubljana, the utility JP VOKA SNAGA says drinking water is chlorinated only locally and occasionally: in its September 2026 notice the Kleče supply zone (138,430 users) has no disinfection, while the Brest plant uses chlorine gas. In Zagreb, the utility's August 2026 results for the Zagreb South zone show free residual chlorine of 0.06 to 0.37 mg/L, above the level MSD describes as harmful to fish. Because supply can change, use a conditioner every time.

Tap water can also contain nitrate. Slovenia's drinking water decree allows up to 50 mg/L, which is why OATA defines its aquarium limit relative to the tap water level.

How to change the water, step by step

If your tank is only a few weeks old, read how to set up your first aquarium before your first changes.

Warning signs of poor water

• Fish keep gulping air at the surface. The RSPCA treats this as a sign of oxygen shortage or poor water quality; labyrinth fish such as the blue gourami are the exception, because they breathe air naturally.
• A test shows ammonia or nitrite above 0 mg/L.
• Nitrate keeps rising week after week despite water changes.

The RSPCA advises testing the water at least once every 14 days, and straight away whenever something seems wrong.

Frequently asked questions

Is it enough to let tap water stand overnight?

Not reliably. Free chlorine does slowly escape from standing water, but according to the University of Florida chloramine does not gas off as easily, because the chlorine is bound to ammonia. Since you rarely know exactly what your supplier uses or when it chlorinates, it is safer to add a conditioner that binds both chlorine and chloramine to every batch of new water, dosed for the volume you add.

Do I need to take the fish out during a water change?

No. The RSPCA method states plainly that the fish stay in the tank while you siphon out water. Netting them is unnecessary and only adds disturbance. Move the gravel vacuum slowly so you don't suck up small fish or fry, and pour the new water in gradually. Only catch fish when you have to move or dismantle the tank, not as part of routine maintenance.

Can I just top up the water that evaporates?

Topping up only replaces evaporated water, while everything dissolved stays in the tank. MSD warns that with this kind of maintenance organic acids build up, gradually lowering the pH and slowing the biological filter. Nitrate stays in the water too, because only a water change removes it. Topping up is therefore an addition, and regular partial water changes remain essential even when the water looks perfectly clear.