1. undergravel filters
undergravel filter (under)
Undergravel filters are one of the oldest filter systems. But with undergravel filters you will probably need a external filter because undergravel filters do not have a place to put activated carbon wich most fish tanks need.
2. Power filters
Power filters are a kind of hang on the back filter They have and inflow tube were water from the tank is sucked up , the water then passes through a carbon which should be replaced every few months
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