I agree with Bob. I have 2 in a 125 for about 6 months now and they are all over the place. They regularly swim to the surface at feeding time and have great personalities. I can't imagine they would be happy in a 5. Maybe a very small one with some rock to play on?
I think that if you provided a cave or rock it could hang upside down on it would do fine. Even 2 IMO.
The problem is that they need quite a bit to eat to stay healthy so water pollution could be a problem. Frequent water changes would help with that.