The water change is the most basic and essential part of aquarium maintenance. But can it harm or kill fish?
Waste easily accumulates in poorly and irregularly maintained aquariums. This waste is visible as mulm in and under the gravel. Waste also accumulates in corners inside the filter, and in tubing. Further, waste collects inside the aquarium, on and under decor, ornaments, rocks, driftwood etc.
If a partial water change has not been performed in a long time, a large water change will instantly and dramatically alter the chemistry and balance of the aquarium.
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Some waste, especially in low oxygen areas, is toxic and can cause fish to die in a very short period of time. Toxic conditions severely stress aquarium fish, resulting in a vulnerable immune system. Bacteria and parasites are always present and will prey on the weakened and vulnerable fish. Dissolved waste creates poor water conditions. While these conditions are not as lethal, they will stress and negatively affect fish over time.
So the answer is yes… if a water change results in a sudden and dramatic change to the water chemistry… it can have severe consequences and can even kill fish.
If water changes haven’t been done in a while, or the tank is heavily polluted for any other reason, then you should make small incremental changes to bring the tank back into balance. A 5% change combined with siphoning of the gravel can be done twice a week. This will allow fish to slowly acclimate to the changing conditions.
Also see ‘The aquarium water change‘