By "seasoned water" I mean, water that has been left to stand or treated in some way to remove chlorine and chloramine that councils add to water supplies to make it safe to drink.

I'm wondering with the new info you provided if your tanks are/were cycling and maybe the older fish tolerated it but the fry and the female who delivered couldn't handle it.

Your water changes are good - keep that up although I'd decrease the 10gall to maybe once or twice a week. Umm, are you just changing 20-25% of the water via a gravel vaccuum? It is not necessary to change all the water and gravel vaccuuming with a cheap plastic device (syphon) helps remove build up of deitrus in the gravel.

Water quality is the key to success apparently. Fish can fight off many diseases if they are not stressed by problematic water. Howeveer fry are maybe less likely to cope with disease than adults.

Cycling of a new tank generally takes 4-6 weeks but it depends on many factors such as water changes. I'm a bit short on time right now so if you don't know about cycling a new tank check out the QuickLinks.

My fry only got powdered flakes until they were big enough to eat freeze dried bloodworms as well. I believe that should be sufficient (even if not perfect).