Aquarium water changes: how much, how often and how

Regular water changes are the most effective thing you can do for your aquarium. They remove nitrate and dissolved waste, replace minerals and trace elements, and keep water chemistry stable.

How much and how often?

A good starting point for most tanks is 20–30% once a week. Adjust it to your tank:

• Community freshwater: 20–30% weekly
• Heavily stocked or large fish (cichlids, goldfish): 30–50% weekly
• Discus and other sensitive species: 30–50% twice a week
• Planted tanks with fertilizer: 30–50% weekly to reset nutrients
• Shrimp tanks: 10–15% weekly; shrimp dislike sudden changes
• Reef tanks: 10–20% every one to two weeks

The right amount for your tank is whatever keeps nitrate in the target range between changes.

Doing it safely

1. Match the temperature of the new water to the tank within 1–2 °C.
2. Treat tap water with a conditioner that removes chlorine and chloramine before it goes in.
3. Match hardness and pH. If you mix RO and tap water, keep the same ratio every time. For shrimp, match TDS as well.
4. Siphon the substrate while you drain, especially in open areas where waste collects.
5. Refill slowly so you don’t disturb the substrate or stress the fish.

Saltwater

• Mix salt with RO/DI water and let it circulate and heat for at least 12–24 hours before use.
• Check salinity with a calibrated refractometer; aim for the same value as the tank.
• Top off evaporation with fresh RO water only, never with saltwater, or salinity will creep up.

How big can a single change be?

Large changes are safe if the new water closely matches the old. Problems start when the two differ a lot, for example a tank with very high nitrate or a pH that has drifted. In that case, bring the values back over several days with 20–30% changes instead of one huge one.

Common mistakes

• Skipping changes because “the water looks clear”; nitrate is invisible.
• Using hot tap water; it can carry metals from pipes.
• Cleaning the filter and doing a big water change on the same day in a young tank.
• Changing a lot of water in a shrimp tank at once.

Fishtify turns this into a schedule: it sets a water change task for each tank, reminds you when it is due and shows exactly how many liters to change to reach your target values.