Why does my fish attack the bubbles in its tank?

I just won a small fish (type unknown, from WalMart i think) over the weekend at a carnival and I bought it a gallon tank with built-in filtration and aeration or whatever (seen those spongbob tanks?) and it has been really happy until today, when it started attacking the bubbles and freaking out. What should I do? Could it just be its reflection at the back of the tank?