Depends on the size of the pair and the amount of dithers. I had a small pair in a 40b with probably 30 buenos aires tetras and no losses. Pulled 25 half inch fry out. The pair stayed near the bottom and kept their fry on the bottom as well.
Overwhelming numbers of threats keep them from using lethal force as they know they can't possibly single out that many threats, so they just focus on keeping things away from directly attacking fry. Larger pair may have an easier time picking off dithers.
Ime the best dither is just other convicts, like 4 additional ones from the pair. Convicts I am starting to find aren't exactly pair fish. They are made to live in big colonies where they have the opportunity to swap mates when things go wrong with one.
