well, nothing. your gold fish is going to get too big for your tank. plus it makes a HUGE amount of waste. get rid of that fish asap, or get a bigger tank.
once you solve that serious problem..you cant get much in a 10 gallon tank. how about a betta? or how about 3-4 gouramis? a school of rasboras? maybe a school of some small tetras?
like i said, 10 gallons is really limiting it, and if youre going to keep that goldfish in there, dont get any fish. not only cause its gonna get big, but goldfish like different water temperature than other fish.