Hi, have a tank about 4 ft x 3ft x 3ft in the garden in north london u.k.
it overflows into a much smaller tank. last year i put two goldfish in the
large tank. later small black fish appeared in the small over flow tank, but
interestingly none in the big tank. (guess all the eggs must have flowed
into the small tank) i am wondering why no baby fish appeared in the big
tank though?

recently i took a few of these babies out of the small tank and put them in
an indoor glass tank. a couple of these have turned gold colour, but the
ones outside still have stayed black even though they are bigger than the
ones taken inside. why is s this would you think? what in fact
determines when they change colour?