I agree the tank is overstocked, and the first solution is a much larger tank.
That said, convicts are some of the best parents in the fish world. They will defend the nest from just about anything. The easiest method for fry survival, and the best imo, is to just let the parents take care of their spawn.
Now, in so small a tank it can get tricky since there isn’t much space to go around. The cons will either eat the fry because they feel it is unsafe, or they will start making space by removing the competition. To calm aggression and ensure fry survival, you can pull the eggs/fry and raise them yourself, but that can be a bit trickier (I have no experience there).