Backyard Aquaponics
A place to discuss aquaponics

RB, I've kept fish nearly all my life and never done an autopsy. (And I assure you, I've killed my quota of fish.) Hang tight and just watch the fishies. They will teach you what is normal. AA gives some good descriptions of the behaviors to watch for.

If you do end up needing to replace fish, you might consider spluging a bit and using fish that they will let you pick the individuals. I know a lot of places here will sell feeders at 10/$1, but there's no selecting them. They just scoop them up and count them out.

You are seeing more golden color because they are maturing. Many goldfish start out drab, and then color up as they mature.

The signs I look for are flashing, gasping, erratic swimming, rubbing, and "ghosting" or when they turn pale - a sign of stress. If they are healthy colored, swimming normally, and a normal, fat belly, then I don't know why they are dying. When they die, do they die with a rounded belly, or pinched belly? Pinched belly is sign of problems too.