In a 15-20 gallon tank, I would go for one of the Pseudomugil species. There are several species to choose from and some prefer soft water, while others prefer harder water and some even brackish.
I have honey blue-eyes and they are truly spectacular when displaying and sparring. If you want them to spawn, all you need to do is add a spawning mop or a handful of java moss, give them lots of live food (I do blackworms, grindals and BBS) and they will usually do the rest. They are egg eaters so you need to comb through daily and remove any eggs for artificial hatching.
In a tank that size I'd recommend a school of delicate, spotted or forktail blue-eyes depending on your parameters. Forktails and delicates are more suitable for harder water and higher pH, while spotted and honey blue-eyes do best in softer and lower pH water.
Here's a picture of a happy 'frisky' honey blue-eye male
You could also try a small school of Melanotaenia maccullochi might work in a 15-20 gallon tank. They don't grow very big, and the Skull Creek variety is absolutely stunning.