Bill, but the survival of the fittest would in my opinion also be the survival of the quickest, – so any ‘too slow’ fry is also ‘eliminated’ just as well as a fry with problems. I dont really think that a female would be able to determine if a fry has any physical problems or deformity, but simply having that means that the fry is probably less quick and less able to protect itself.
But I guess having this in mind could indeed mean that by eating own fry it will be the quickest and strongest that survives.
But I must also say, I am thinking of the difference between a dark, deep flowing water somewhere in the rainforest with lots of leaves and dark water, – I think probably quite a few more fry will escape than in our small tanks, where theres a limit to how far they can escape.