I have had swordtails for several months now and they are a joy to have but I must warn against the aggressive tendencies of some males. I first bought 2 males and 4 females and thought that was a good balance, but the smaller male was killed by its rival within days of introduction in my community tank. I later introduced another healthy male, but though he lived he lost his long tail in a fight with the resident male. Probably the loss of his tail makes him unattractive to females. Also I found my bully to persecute one of the pregnant females, after unsuccessfully trying to mate. This may be just one isolated "black sheep" but his sex drive makes him quite a nasty fellow. Other than that my swordtails get along with everyone else in the tank, which harbors 13 different species from 4 different continents. I find the key to such a successful community is tank size (470 liters) with plenty of hiding places (rocks, plants) at all levels of water, so that each species has their favorite home and do not compete.
I have about thirty swordtails of both sex. Each one is a beautiful common fish, they stay together in my 230 L fish tank and swim happily together. But because I have so many I always have to breed them and have to always pull more than one pregnant fish out of the tank and put them into a breeding tank at the same time. I have 4 breeding tanks which are mainly use for the swordtail fish. They are my favourite fish in my tank including my loach and catfish plus more. They make the tank colour up with orange plus the other colours of my other fish.
These fish are seriously good jumpers. I had a problem with a few of the getting Ich. I put 10 of them in a 55 liter hospital tank. There was about 10 cm from the top of the water to the top of the tank. When I got home 6 had committed suicide. So I made a lid with only a Ĺ hole in it for air tubes, ect. I was pretty surprised to find the rest jumped out through that. Thatís pretty accurate Iíll say.
I have a few swordtails from which I just need to mention two, that have interesting characteristics. One is a (quite dark) pineapple color female which is a massive 11 cm long (above normal), quite extraordinary. I have not attempted to breed her to see if I can keep this genetic trait in her offspring. She is non-aggressive but very curious - ask my golden snail, he has being feeling like a football lately. The other is a male who has a lovely color. He is a very dark blue (almost navy) with a green velvet shine. I have not seen many of this color and think it could also be worth mentioning.
I have a number of orange and red rainbow swordtails and now have the fourth generation of fry swimming among the grass and adjacent columns of plants they favor. They truly are wonderful fish for a community tank and breed prolifically in my 227 liter tank with plenty of cover at all levels, rocks, different light levels, and medium currents. I've found a male/female ratio of 1/2 to 1/3 sufficient to keep them happy, but the males will still keep a strict hierarchy no matter how many females are present from what I've seen. Two points: first, predators such as black ghost knives can whip through two dozen or more of the young swords up to 3 cm long in a fortnight (there went all but three of the third generation). Second, both the males and females were aggressive enough to repeatedly attack 3 cm snails, prevent them from surfacing to breathe and eventually tear them up and eat them, leaving only the shell and the foot covering! I returned home after one such feast to find my splendid red male rainbow sword puffed up like a golfball from their second snail meal, he died in a few days. Otherwise they are fine with the Danios, Gouramis, and Plecos I keep and yes, keep the lid tight, including a lid on the outboard filters which these and other fish such as plecos will swim up into. First aid for a suffocating fish? Pull gently backwards through the water to push it (and hence air) through its gills a few times if the fish is already sluggish when found, this should perk it right up.
I just returned two great looking pineapple swordtails to my LFS. One male and one female were added to our tank along with a pair of Turquoise Gourami's and a pair of Kissing Gourami's. Within three days one of each of the pairs of Gouramis and a 5 year old Angelfish were. The LFS wondered if it could be another fish nipping at it and then it becoming infected. I said that I watched the tank and didn't see anything. They recommended turning off the lights and watching from aways away. Sure enough that night as I watched from 20 feet away as both of the two swordtails repeatedly chased and attacked the remaining Gourami's and Angelfish in my tank. I thought they were a peaceful species, but these two were not! Back to the store for them...it's unfortunate because they were good looking, but I'm not ready to risk another set of Swords again.