Tank water is contaminated by food waste left by fish and eaten. So, lets purified by passing through the filtration material of the filter by circulating water. This not only takes over the dirt just physically, bacteria decompose the ammonia in the filtration material to breed, have the effect to be harmless.
The gravel material is activated carbon filtration nylon wool or glass fiber. There are boulders such as the power to circulate the water pump or motor pump is used.
There are different filters in the model filtration efficiency, 100% remediation of any type is impossible. Try to think too much filter aid and reliable.