We have 2 fish, one is a goldfish and the other is a fantail, when we bought them 6 months ago the fantail had black markings on its (how do you sex a fish??) fins and tail, now it is gold all over, no black markings at all, the goldfish has always been gold but now looks like he has lost some scales on his side, we dont see them fighting so presumably it is something we are doing or not doing that is causing the problem? we feed them fish flakes, and when we clean out the fish bowl we do not use any type of detergent and we add drops in to negate the chlorine, any advice grateful

I've always been told black goldfish change colour as they mature, as for sexing them I haven't a clue always been told its quite difficult especially with youngsters.

But a link explaining Free Information on Sexing Goldfish