I have decided to breed Zebra Danios...but I have ?s...

The differences are subtle. according to my "encyclopedia of tropical fish", "the difference between the sexes cannot be seen until the fish are mature. Then the male fish are thinner, more elongated in shape and also more brightly coloured than the females" I wouldn't worry if you have 6 though. the chances of them all being the same sex are one in 2^6 or about 1.5%

good luck with the breeding.
 
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