I heard that if you have two female swordtails one will change to a male is this true?
No. Swordtails cannot change sex as far as I know. Here are some internet discussions on the topic.
http://badmanstropicalfish.com/mb/mb177.htmlhttp://www.aquamaniacs.net/swordtails.htmlA few sites say they can change sex. I really doubt it.
http://animal-world.com/encyclo/fresh/live.../swordtails.php i looked at the sites they were very informative but how do you know if they are females or late blooming males?
In time with maturity and a male present, almost all females will eventually have babies. The males in hiding usually will eventually develop their gonopodium. If not, they will not breed. Also, you can tell mature swordtail females and males apart by more than just the presence or lack thereof of a gonopodium or gravid spot. Females are larger I think and have more round bellies (even if not carrying young). Males are often more colorful and have fancier finnage. Since I've never had swordtails, I tried to search for more info on the internet and up comes my own site! I don't even have any real info on them, go figure. I only looked a little but it confirmed what I said above.