I vote Pygmy cories if you get sand. :) my little guy is pretty funky. He's all alone but he zips around the tank and swims around the middle, and seems to enjoy where the filter pushes him down. Of course you need a large groups they only had the one when I was there, but I'm getting more this weekend :D
Pygmies tolerate a mid range pH and hardness, but some fish are more sensitive. Many amazon fish need soft water to thrive.
Is this your 15 gallon that already has 7 neons?