Why your aquarium needs water changes

Young fish

Water quality determines the continued health and growth of your fish. Water changes, by replacing a part of ‘dirty’ water with an equivalent volume of clean water, reduces concentrations of undesirable substances in the aquarium.

In an established tank, nitrate and phosphate are the two main pollutants that accumulate over time, the former being toxic in high doses to goldfish (although it is increasingly suspected that prolonged exposure to moderate concentrations also has a negative long-term impact, particularly on growth, reproduction and fish health in general), the latter not being toxic but encouraging algae growth as the former. In short, regular water changes are essential for fish but also to keep substances that may encourage algae in the aquarium under control.

How to eliminate chlorine?

Chlorine in tap water is toxic to our fish (burns fish gills, among other things) and also destroys the beneficial bacteria in the filter.

So, to neutralize chlorine, add a dose of water conditioner to the replacement water.

Quality water conditioners (available in pet stores) will also neutralize any heavy metals that may be present.

And some water conditioners also contain plant extracts that are intended to reduce fish stress.

Should I change all the water at once?

Water drop

No. While reasonable and regular water changes have a positive effect on fish and the aquarium in general, brutal changes in the water parameters are stressful for fish and can ultimately encourage the development of disease (the fish’s immune defences being weakened by stress).

Water changes are therefore always partial and carried out with water at approximately the same temperature. Changes in water with too cold water, for example, can trigger white spot disease. No need for a thermometer: test the temperature with your elbow (the hand being a poor temperature indicator).

It is always better to change less water at a time but more often, than a lot of water at once but not often.

When to change the water? And how much water?

Most fishkeepers make a weekly change. The question of the proportion of water to be renewed depends largely on the size of the aquarium, the number and type of occupants (fish and plants) and the quality of your filtration. It is therefore impossible to answer this question as it stands, since every fish tank is unique and particular, but, for example, a water change of 15% to 25% per week  for a medium sized tank seems fine. In principle, the quantity of water renewed is proportionally higher in a small aquarium than in a larger tank. Similarly, the more frequent the changes, the less water is required.

How to clean the filtration media?

Filter sponge mediaWater changes are the ideal opportunity to maintain the aquarium (clean the glass, check that the equipment if working fine, etc.) and if necessary, to clean the filter sponge (or any other filter media your filter may allow). The filter media needs a quick clean when it becomes saturated – this can easily be seen when the flow rate at the filter outlet is reduced. Tap water should not be used to clean the filter sponge because tap water contains chlorine that would kill many of the essential beneficial bacteria present in the filter media. Instead, use water from the tank to give the filter sponge a quick wash. And when the filter sponge becomes worn or porous (usually after several months or even years), cut the sponge in half and change one half first then the second half a couple of weeks later. The old sponge will then have the time to colonize the new one and enough beneficial bacteria will be preserved.

Should I be doing water changes while my tank is cycling?

If you’re using the fish-less cycling method (more info here), not necessarily: you may wait until both ammonia and nitrites have dropped to zero and nitrates have risen before doing your first water change.

But if you already have fish in your tank, please do partial regular water changes. Performing regular water changes is crucial to the health of the fish and, in any case, beneficial bacteria colonizes surfaces like gravel, filter media and plants, so this will not stop the cycling of your tank. Always use a quality water conditioner (dechlorinator) when doing water changes.

More information about tank cycling and the nitrogen cycle here »