My english is bad, but I'll try to answer
On my opinion, the problem are the convicts
When the fry are starting to swim, they are not using all the space in the aquarium. They are usualy all still together, and if they are going on the right side of the aquarium, the parents are follow them and they can protect them. If some fry are going to far from the others, they are going to pick them with theire mouth and release them with the group. But with the time, fry are using more and more space in the aquarium and the parents are then unable to keep them tpgether, and at that time, the convicts have the possibility to eat them when they are going not far from them. When the parents are loosing the control on the fry, they prefer to eat them. The only reason why I am keeping my fry alive is because they are alone in the aquarium with the parents