Really random deaths

Hey,
So I've had a couple of late bloomers in my current batch who were growing very slowly so i put them all in a tank together. Some of them were growing really well and had almost caught up to the others so i was thinking of giving them their own tank, but this morning when i woke up all the big ones (approx 10 out of 25) were dead???? this is more deaths than ive had up until now and it's really puzzling. All the little 'runts' are fine but the bigger 'healthier' ones from that tank popped off??? water parameters are ammonia 0 - 0.25( hard to tell but still should be fine) nitrate 0 nitrite 0 temp less than 18C and pH neutral.

I know they say to expect a fair few deaths when raising larvae but i've never really had any problems like this until now (3rd batch). The ones which died were about 2-3 inches long and coming on three months old.

anyone have any theories? i would like to prevent this from happening again

That is so sad, hope someone can advise you as to what may have been the cause.

4 more since then, must be bacterial or something - I've taken them all out of the tank and put them into isolation. It seems to only be targeting the large ones???