Au, this is a very common phenomena, many fishes exhibit colours or intensity not usually seen under aquarium light. Many of my licorice gourami for example, show fairly pale colours but the flash photographs turn out to be briliantly coloured. I believe this is because camera flash is very close to sunlight in composition.
Also when flash is used, there is usually little need to colour correct, whereas without flash the green cast from plants and glass is quite strong and this needs to be compensated on the pictures.
Bookmarks