gosh 2000, sounds like quite a few batches of fish rather than a one off. tell us a lot more about the set up, the water, photo of tank if poss.
do you have plants (they help keep nitrates and other salts down for example) some plants also give back up food if fry cant get to main supply. the fact that i get nibbled leaves on my soft plants tells me when I am not spreading the food about enough.
do you give any other food besides courgette and flake? maybe more variety is needed, squashed peas are very good as suggested above, also green beans.