Cold water fish

[carl.badham]

Why do cld water fish chase each other around the tank, possibily biting the under side of their bellies?

[wildwood]

Could be that the female is releasing pheromones in the water and this drives the males crazy. If you're in the UK, it seems a bit late in the season for that though - unless the tank is inside where it is kept warm by the house's heating..

[RALLYMARK]

maybe they have nothing else to do. try putting some toys in their tank so they have something to play with

[B00]

I'd go with wildwood's theory- your fish are more than likely just rampant. Mine do this to each other occasionally, but each others bellies, it's how they get the females to release their eggs.

No need for toys, they aren't children needing distractions. Anything you drop in the tank they'll just think it's food, they won't be playing.