my bf has a 29 gallon tank with one of my bettas in it....we have successfully housed danios, numerous tetras, coridoras, siamese algae eater, blue rams chiclids, white clouds. They are all still in there together but I would suggest fish that like different water levels for territories. The betta we have loves the top so we chose middle dwelling and bottom dwelling fish. Nothing that would impose on the bettas personal space...well the white clouds do like to swim around him but they have been with my friend's betta since they were juveniles and then with my bettas for 5 years now and leave him alone. It all depends on your betta, Remus is a very peaceful betta. One of my crowntails JD he hates any fish in with him...he hated my poor cori and beat on him so I had to take him out and put him in with Remus' tank.