Too many bubbles from airstone??

Well, I have two small goldfish in a 5 gallon tank. I know I know,
thats not enough room but thats all I can afford at the moment.
Anyway, I bought a 15 gallon air pump (i'm looking into getting a
bigger tank) which is attached to a filter, and and airstone. The
airstone is blasting out bubbles and its making the top of the whole
tank all bubbly/foamy. There is hardly any surface water that isn't
bubbly. Is this normal or do I need to do something? Thanks for any
help.