I have had about 6 of them and mine did the same thing. I had them in a 40 gallon octagon tank and all they did for more then 6 months was circle the tank day and night. They are crazy!! Now they are in my 100 gallon with freshwater and African chiclids and they are fine!! I only have 2 now and they are very active but on the bottom of the tank and love a huge catfish I have.
This the catfish they always hang out with. He is about 5 inches bigger now but they love him even though he barely moves.