Koi Swordtail Couple
I keep some bright orange Swordatils (2f, 1m) in my Amazon biotope with various schools of tetras & two types of dwarf cichlid & several different Corydoras. They make an excellent addition, they are non-aggressive to the others in the tank and they do a great job of surface skimming, eating the food that doesn't sink to the other fish. They breed regardless of the pH and are less common than Guppies or Mollies so the fry tends to be welcomed by my LFS. Swordtails are a good all-rounder.
I have found the Xiphophorus hellerii to be a most interesting species. They are a most interesting and charming species, whose brain capacity should not be underestimated. I have personally trained the female swordtails to collect floating waste from the tank and deposit it at the site of the filter. Quite fascinating indeed. It is wonderful to see that there are people who appreciate these remarkable little specimens as much as myself.
I recently bought a trio of swords and they have bred readily in my aquarium. Perfect for beginners to breeding!
One thing worthwhile to note about Swordtails that I haven't seen mentioned by anyone else here is that, more so than most other livebearers, they can be exceedingly enthusiastic jumpers. I had my tank carefully closed up, having learned this particular lesson the hard way, except for the one necessary opening for the return flow of my older style power filter. One of my swords, apparently thinking it was a salmon, managed to swim/jump up the waterfall into the top of the filter, from which it was an easy jump down to the carpet. I don't know how long he was down there, but he managed to revive...only to do exactly the same thing a short while later as I was still working out how to effectively enclose this loophole. Gotta give him points for persistence...and subtract a few dozen for failure to learn anything the first time...but he did survive the 2nd jump.
Swordtails are the hardiest breed I have ever kept. I still have one robust female from years ago, who survives no matter what; while all the other fish get sick, she justs swims around happily.
Reading a lot of the comments about swordtails really made me chuckle - I didn't know they were an easy-going breed! I knick-named our 8 females "The Hussies!" They are constantly bothering the other fish in my tank - nipping at my 23 cm pleco, corralling my neon tetra, etc. My anglefish turned flat mean and now chases them after they tore his fins to shreds (he's more beautiful than ever now). I think because of those swordfish, my whole tank is more aggressive than it should be! Oh, and the male (my husband named him "Lucky"...), he just hangs out of the way under an amazon sword leaf most of the time - he's the only peaceful one!