Xiphophours monticolus is a beautiful and recent wild swordtail import. It has commanded fairly high prices at auctions likely because it has proven fairly difficult to breed and raise. They are well known for their sensitivity to water conditions and their ability to eat their fry. After losing these fish at least 2 different times, I finally succeeded in raising some fry, and thought I would pass on what was learned to others. The setup was a 15 gallon tank with a dolomite undergravel filter to increase the hardness of the water. At one end of the tank was an incandescent bulb for growing plants and at the other end was the undergravel filter with very low light conditions. A few potted plants and some weighted anubias and floating hornwort were in the lighted end to give fairly thick cover.
These fish were very shy at first, so I added 2 pairs of Scolichthys iota, a new small but active livebearer as dither fish (I first checked with Pat Hartman to see if he felt they might crossbreed, and he didn’t feel that they would). At first, the X. monticolus stayed in the darkened end of the tank and did not feed at all. Some blackworms, daphnia, and the dither fish helped bring them out and feeding within several days. The first fish to have fry was Scolichthys iota, and I left them in the tank. They are small but very quick and could escape the monticolus trying to eat them. When the monticolus had fry, there were enough Scolichthys fry in the tank to distract the adult monticolus so most survived. The swordtail fry were also very small but slower, and at first they stayed at the bottom of the tank where they were more vulnerable (the adult monticolus were reluctant to feed off the surface). Keeping them well fed helped, and they became accustomed to eating heartily on flake and frozen foods, as well as live foods. A spirulina flake was included to give them vegetable matter. When I started, I had 2 males and 3 female monticolus. The smaller male was dominated and prevented from eating by the larger male and eventually died.
Xiphophorus monticolus is a beautiful and challenging fish to breed. I hope that my experience will help others enjoy and be successful with this fish also.