Probably the fish are fine if they eat an ant or two, but it made me think back to a few years ago when I fed them to my pet toads. They were American toads that I'd raised from eggs and since they only see food that is moving I had to provide them with live food. Not easy.
I did the mail order crickets, meal worms, earthworms were always popular although they were a bit of a challenge for the toad because they kept wriggling back out. Very creepy the first time I saw it.
Anyways with the ants, which were easy to find, the toads snapped them right up but a few seconds later they seemed stressed and suddenly out came the ants, all agitated and dashing around and completely un-harmed. After that the toads wouldn't go near them.
I thought it was odd but then I remembered how when I found an ant in the bathroom I grabbed it with a piece of toilet paper and threw it in the toilet.(Not nice, I know, it was just knee jerk reaction). The ant continued to dash around and fidget underwater, seemingly undisturbed by being underwater and continued to do so even as I flushed. (I felt really bad after I did that--I was just so creeped out)
So it may be with the fish, as I think with my long departed toads, (and being flushed), that perhaps ants don't die right away and may sting or whatever even as they are going down. They're such tough little insects. I don't know if they would do any harm to the fish or not.
Nothing scientific here, just a personal observation--
Robin