Hi Marc,
Sorry to hear about that, they were looking fine too.
Anyway, I would be suspicous of the courgette introducing something to the water. I have heard various stories of supermarket bought produce having this effect (pesticides etc etc etc) but not really given it much attention as I feed my (adult) fish this sort of thing all the time. That said, it might be more prone to affecting young fish.
Are these in the main tank or one of the smaller ones? Given the smaller tanks are quite new I would also be a bit worried about feeding veggies in that size of tank. You can wipe out a smaller tank real quick with feeding veggies that are not eaten quickly. In this case I would do lots of large water changes from your larger tank and refill that will fresh water.
Jools
PS How are my fish doing?