Will be fine, I am looking at getting Killifish just for that reason, to feed them top water food and watch them go after it. Make sure to get the flightless or wingless ones though and D. Melanogaster species because they are smaller and more prolific, and easier to culture, if you don't have them already. Hydeii are ok but the cultures are slower and they can crash more readily because the food isn't eaten as fast to prevent mold outbreaks.
Well I should ask then what species of fruit fly do you have, if you know and if you have one?
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