youngsters dying after 4-5 weeks

[cloudies]

About 5 or 6 weeks ago my group of C121 gave me the responsibillity over 30-40 eggs, since that day I have been treating them like they were my own... after a small week the babies that came out of these eggs and they grew rappidly. After 3 weeks around 25 youngsters survied my care!

From around the 4th week they start to die one after another... each day 1 or 2... Today (they are now 5 weeks old) I only have 4 youngsters left. I did not change anything to the water (except ofcourse normal water changes).

What could have caused this? By the way, it looks like the fish that are dying are the biggest ones, the smaller ones keep living the longest.

[syno321]

In prder to help I think more information would be needed, such as foods, tank set-up, water parameters, etc.