Hi Craig, yes I have had this happen to me before, last year on some fry around a month or so I lost 4 or 5 toal from 4 spawns which I had around 40 fry, these had been placed in 2 fry traps.
I was losing the odd fish every few days & all fish looked fine & healthy.
This year I was lucky to have my first spawn in my big 700l tank that I had been waiting for a spawn for 2 1/2 yrs

It resulted in 15 fry that I put into a breeding trap & again around a month old I have lost 3, these are 2 months now & doing fine.
I can't answer whats happening I put it down to nature & they may have had some internal problems
But once I get past the month stage they always seem fine after that.
I guess that if they where left in the main tank we would rarely see any dead babies as its a lot harder to count or see them there compared to a breeding trap.
I normally return fry back to the breeding tank to grow on after 3 months & I only placed my new babies in a trap as I have FX5 filters that would suck babies up, I will put these babies in another tank with a smaller filter.
