02-02-2010, 05:52 PM
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If you recently purchased the female she could have stored sperm from male double tailed guppies, and gave birth to his babies. If you've had it for several months it probably won't have anymore stored sperm from the fish store, so it should be only having the babies of the male's you have in the tank. I don't know very much about genes, but if double tailed is a recessive gene and both of them have that gene in them, then you might get some double tailed guppies. If you have a large amount of fry and only one of them looks like this, it most likely has been bitten or something. Also, if you believe it's a female, I actually haven't seen a double tailed guppy female, but I assume that it would have a way less noticeable double-tail.
Lastly, all livebearer fish fry at first look to be females. I used to wonder why I only got female guppies when my guppies gave birth. You'll soon see that as they mature the males will start getting more color, and will develope the gonopodium. When they are that young it is impossible to sex them.