You could try slowly increasing your feedings until they no longer find interest in your frogbit. But be mindful not to overload your system with nutrients in doing this as you don’t want to bring on an algae bloom. If this doesn’t work you could add another species of floating plant like duckweed.
Be cautious with this, though, as duckweed can very quickly take over and smother out other species of floating plants. I have had an aquarium of guppies numbering in the 30s that survived solely on duckweed and organisms that lived within it.
They will appreciate the occasional feeding of baby brine shrimp as well.