>There is not enough oxygen in the water deeper in the tank so they are coming to the surface. Highly oxygenated water is usually higher in the tank.
> there is not enough water movement at the bottom of your tank so the water chemistry at that level is not to their liking.
> They just like the way water feels as it blows through their hair ( shells) lol
(I'm sure there are other possibilities as well. Just can't think of more at the moment. Check your levels at the bottom of the tank
Hope this helps