Water Changes

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Water change 800.jpg
Comparison of water parameter stability with different water change regimes.


Water changes are useful for diluting substances that accumulate in the water, such as nutrients, and for replenishing substances that get gradually depleted, such as magnesium. Water changes are generally not useful for maintaining calcium or alkalinity as these are depleted too rapidly.

Are They Really Necessary?

Many hobbyists manage to get by with only irregular water changes or none at all, however, as it is not easy to measure all the constituents of water in the tank, it is not known how far and how quickly the make up of the water diverges from natural seawater. Regular water changes help to reduce the unknowns involved and drag the water in the aquarium back towards natural seawater, at least partially.

How Much and How often?

This question is asked quite often and there is no absolute answer. The larger the water changes and the more often they are done, the closer the water will be to natural seawater. However, frequent large water changes are usually not practical or sustainable (you'll get sick of it very quickly) and so it is best to work out a manageable regime.

Replacing around 25% of the total volume of the tank every 4 weeks is a good goal. If you can change more in a 4 week period, then even better. If you can't change as much, it may not be as good, but will be adequate.

From a long term dilution and replenishment perspective, it doesn't make any difference if you change 25% once every 4 weeks, 12.5% once every 2 weeks or 6.25% every week. Larger more infrequent changes may have more impact on the inhabitants of the tank because the changes in water chemistry will be greater, but even 25% water changes should not cause any problems. It is best to choose a regime with which you will be comfortable.

Match Specific Gravity and Temperature

It is important to try to match the specific gravity and temperature of the new water to that of the aquarium, especially if large water changes are being performed. Even if they don't exactly match, it is important to understand the impact of the differences. For example, if the aquarium water is 25ºC, the new water is 15ºC and you change 25%, the resulting aquarium water would be 22.5ºC which is probably too large a drop in a short space of time. However, if the new water was 20ºC, a drop of 1.75ºC to 23.75ºC while not ideal, would be tolerable.





Which is better: 1% daily, 7% weekly or 28% every 4 weeks?

There is no long term difference between doing daily, weekly or monthly water changes if the total amount of water is changed over the period. For example, if water changes are being performed to dilute nitrate and nitrate is being produced at the same rate, the long term maximum concentration of nitrate will be the same for daily, weekly and four weekly water changes, if the same total amount of water is exchanged each 4 weeks.

The graph below is an example of a tank where nitrate is being produced at 0.1 mg/L each day and water changes are performed at 1% daily, 7% weekly or 28% every 4 weeks over the course of a year.

Nutrient concentration under various water change programmes.

The exact same trends are shown when replenishment is considered, i.e. long term it make no difference if the same total amount of water is exchanged over a set period.

If you take the organisms into account, frequent smaller changes are probably slightly better as there will be less variation in water conditions with each water change. If the specific gravity of the replacement water is the same as the existing water, even large water changes (up to 50%) are unlikely to cause problems for the tank inhabitants.

Ultimately, it all comes down to personal preference and convenience as to whether very small frequent changes or less frequent larger changes should be performed. Pick a routine that you can maintain and stick to it.