01 March 2018 - Updated 27 March 2019

Changing the Water in your Aquarium

Partial water changes in aquarium

Partial Water Changes In Aquarium

As long as there are aquariums, there will be in-depth discussions about how frequently the water should be changed! Although this opinion is nearly obsolete, there are some exotic fish keepers who believe the aquarium works with a biological balance similar to that found in nature, and that changing the water can damage this.

An artificial system

Theoretically, there may be logical thinking in this idea, but in practice it is very different. No matter how large your aquarium may be, it remains an artificial system that is unable to regulate itself, as it has not been created according to - and nor does it operate to - certain evolutionary standards. In nature, there is a constant metabolism that ensures anything that accumulates is reduced and decomposes back into basic components, which form new life and, thus, the cycle begins again.

Incomplete metabolic chain

In an aquarium, this metabolic life cycle is not complete, meaning that certain products remain unprocessed and build up over time in ever-increasing concentrations. At some point, a limit is reached and your exotic fish will no longer be able to cope – that fish that recently looked so healthy will bid you a swift farewell.

Partial water change

This need not be the case! The best way to care for your aquarium and ensure there is clean, fresh water is to regularly perform partial water changes. How much and how often the water should be changed depends primarily on the size of your tank and the number of inhabitants.

Rule of thumb

A general rule of thumb applies in this case: the smaller the aquarium and the more fish it houses, the more often part of the old water should be replaced with new. The amount of water that should be changed is generally around 30%, or one third of the tank’s capacity. Renewing this level of water every fortnight means that, more or less, you can’t go wrong.

Anyone with fish such as large cichlids from Lake Malawi will want to change around 50% of their tank water weekly, as these aquariums are generally densely occupied.


The actual technique of changing aquarium water also varies depending on tank size, but it is usually done simply with a hose. You can either drain off some of the water using a hose and bucket, carrying it back and forth, or the hose can extend to the deeper drain, with a connection available for a fresh water supply.

Marks around the side of the aquarium will allow you to maintain a constant water level. The temperature of the fresh water being added should be the same as or slightly below that of the water already in the tank.

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