I would be very careful with petshop stock, yes they can get pure stock but most of their zebras will be hybrids some of them may throw relatively true and resemble pure fish but can throw all sorts of rubbish, same with stuff from unknowns on trademe so many are bred in mixed mbuna tanks where all sorts happens.
I would say that the big fish attempted to eat the fish in the net, they suck them through the netting, this mashes them against the net and kills them.. The bigger fish probably add the remaining ones that you transferred.