[Varmit]

I got my daughter a small 5gal aquarium and two goldfish. She picked out the two fish and one of them happens to be much larger than the other. It looks like the larger one has been taking bites out of the tail of the little one. Is that something goldfish do and if so..what can you do to prevent it? Do I need to be feeding them more? Do I need two goldfish of similar size? Help!