Thanks, you've just given me a thought. I've ants all over the garden during the spring/summer/autumn months. They are small, the basic ones that get everywhere. My hatchetfish would love them. Don't know why I didn't think of this before. When I had my frog and newt they regularly received flies and the occasional earthworm. Never thought of ants for the fish.
As long as you don't use any type of insecticide or chemical they could pick up, and I use nothing like that in the garden, I would think they should be fine.