15g for two gouramis and a betta? Anything else? This sounds like a very bad idea to me.

Can you describe "playing"? Because chasing and nipping is considered territorial, aggressive behavior.

EDIT: Saw the picture, his fins are *definitely* nipped. Some aggression is happening somewhere.

Change as much of the water out as you can, as soon as possible, and use dechlorinator when you add new in (at the same temperature of the tank).