You could always use bricks or something similar to lift the ten gallon high enough to maintain the same surface. I think I know what your talking about here. It would be pretty cool to place the ten gallon dead center up front and be able to have the other fish (in the 40 gallon) be able to swim around. Maybe you could make a setup like a split 10 gallon between two boys and a female sorority in the remaining area of the 40 gallon. Impressive Idea! Would love to see what you come up with. Filtration for the ten gallon could be simple by making a home-made sponge type filter or buying a submersible air pump type, and the 40 gallon a regular filter would do. A whole new type of beta barrack! Pretty cool!