There is good news and bad news if you let swordtail fry mix with platy fry. The good news is simple, both fry require almost identical conditions and the fry of both groups will thrive under identical conditions. The bad news is simply stated but a bit harder to deal with, the swordtails and platies are both closely related groups, not really separate species in the conventional way of thinking about them, they will be hard to tell apart and separate as they mature toward a mature fish age. The problem is easy to state but not as easy to deal with. What you end up with is fish that can easily breed with each other that look almost identical. The platy fry, can readily breed with the swordtail fry, and vice versa. Here we go with the issues, how will you know where to place a particular fry in terms of it being platy or swordtail. Swordtails are notorious for being late developing in terms of showing secondary sex characteristics like the sword. What that means, in a practical sense, is that you will have no way of separating platy males from swordtail males. I tend to keep only the pure wild type platy and swordtail types and even in my situation I find that separating swordtails and platies can be problematic. As an example of the difficulty of sexing swordtails let me give my own experience. I had a group of unsexed juvenile fish of X helleri that I got as juveniles. I saw one of the adult looking fish acquire male characteristics at an age of over a year and decided that I had a male among my 6 fish. After a while I was concerned that none of my fish seemed to be carrying fry. A short time later, at an age of about 20 months, I started to see signs of being male in 3 more of my fish. Today, at about 2 years of age, all of my fish seem to be males and 4 of the 6 have swords. The other two have a starter version of a gonopodium but are still not showing a sword at two years old. My concerns over the last 2 years that my fish were not producing fry, while most fish seemed to be females, would have been reduced if I had recognized at the time that those other fish were males. If I had a typical mixed fish derived from swordtail and platy crosses, even the relatively difficult problem that I face would look easy.
They are currently sitting in a breeding trap in my second tank. The only other occupents of the tank are some two week old platy fry.
