I agree with everyone else, that a separate tank with a predatory fish would be the most likely to work.
If that isn't an option right now, you could try very fast fish such as giant danios. They might be able to sneak some fry from the convicts. The convicts also may eat most of the fry themselves if you leave them with the parents; to prepare for the next batch. Any fry remaining after the point which they should have gone off on their own, are considered to be "duds" by the parents, and are usually eaten to recycle their energy. Of course, some may escape this fate, and some parents are more prone to eating the clutch than others.