On the above water parts, I assume? Many species of ants run aphid farms. Aphids produce a waste product that is high in sugar, which ants like to eat. The ants will find existing aphids and even "plant" them on new plants, then protect them from predators. It isn't that different from what we do with cows and sheep. We used to get this on our wild plumbs all the time.
There's no point adding beneficial insects like ladybugs or lacewings, because the ants will just kill them or drive them off. If you want to get rid of the aphids, first you have to get rid of the ants. I'd be reluctant to use DE in a pond if it had inverts in it; it doesn't seem harmful to fish. There are various other ways to do get rid of ants, though; I recommend this one.