It may not be deeper than a standard 10 gallon but there is no horizontal room so where in a 10 gallon a long finned fish can swim in sort of a "z" pattern to go from the top to the bottom a fish in this tank has to basically go straight up. Even in 10 gals filled to the top I say you should have plants toward the top for fish to rest. But like I said for a short finned fish who doesn't have to lug all the extra fin around or a female this tank would be great.
I don't like under gravel filters because they don't have the surface area to cycle.. they're basically pointless IMO. Plus I've heard they can get clogged easily. Also I'm not convinced bettas really like bubblers... all of the fish I've ever had have always preferred the stillest water possible.