11-02-2011, 09:05 AM
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In nature, fresh water is fresh because nature is constantly renewing it with rain. This is especially true in the tropics. The ideal aquarium would have an overflow and a drip system so fresh water was constantly replacing tank water. Not especially practical for most, so we do weekly water changes instead to dilute the pollution in the tank. There are some that believe this water change is just to remove nitrates. However, there is other pollution in the water, including fish urine, pheromones, organics in solution....
How much to replace relative to the bio-load is perhaps the difference between what can you get away with and what is best. Obviously in a larger tank with less stock there is less pollution concentration than a smaller tank with greater stock. But if we err, it's better to have the least amount of remaining pollution in solution.
I have a 60g tank with a growing bio-load due to several young Platys growing like weeds. I try to be conservative with food to prevent over feeding that would increase decay.
I do a 50% weekly water change without fail.
As I do not have live plants to fertilize, this water change often includes an aggressive gravel siphon to remove decaying organics. I'm sure my fish are better for this than if I was to do less.