For my betta tanks (see here--> ), I purchased these corner filters: I've tried using plant sinkers, aquarium gravel, and rocks...they still try to float! You can see in the 2nd (purple) tank that I've got aquarium gravel in the bottom of it and a rock on top of it. I've also got 3 plant sinkers dropped in the spot where the bubbles come out of the filter. I don't know what else to do! The filter in the green tank isn't much of a problem, but the one in the purple tank (and the new blue one I set up today) just continually want to float. The only other thing I can think of is gluing a big rock to the bottom of the filter, but the new tank has a large piece of slate inside the bottom of it and it still wants to float!