Not a good idea at all to feed adult ants to your fish. Most ants can spray any of several types of acids from their mouths depending on species. Fire ants harbor a venomous sting which contains a toxic alkaloid venom called solenospin. While this venom has a short duration before breaking down in the body through histamines, a major ingestion from more than a few ants can cause death. Back in the day before the readily available prepared fish foods, ants were used to feed small and medium sized fish. However, fish were fed on the eggs and pupae, not on adult ants.