Is your 300 gallon tank a gravity feed? If so, they usually put slope in them such that the back of the tank is lower than where the fuel comes out. That way the water settles to the bottom and flows to the rear of the tank where it can be drained (there is usually another plug on that end of the tank).
Do that, drain the water out once in a while, and also filter it like you're doing and you should have no trouble, provided the tank itself doesn't have big leaks where water can get in.
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