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I have a 20 gallon tank and I bought a "Duetto 100" which is designed for "Up to 20 Gallons". I filled the tank about halfway, around 6-8 inches. I read that a general rule is to get filters designed for twice my tank size (In this case, 40). Should I buy a new filter?

You'll probably find out soon enough for yourself. But my suggestion is yes. I currently have one rated for a 30 gallon tank in a ten gallon tank (3X) and I think I could use a bigger one. I have to clean it weekly. I'm not familiar with that brand but someone else might be able to give you a better answer.

Duettos are inadequate as filters for turtles--they can't keep up with the waste.