I have 5 kuhli loaches in my 15 gallon, and I liked them so much that when I got my 30 gallon I got 6 kuhli loaches for that tank too!
They seem to be very hardy, they are great to watch, completely unagressive, and every other fish seems to ignore them too. They are fine with gravel, though rounder gravel is best. In my 30 gallon I have 2mm sand coloured gravel (so it looks pretty much like sand) but because of the way it compacts, they can't bury in it. Also, the more cover you provide them with, the more you will see them, as they will feel more secure. If you look in my signature, you will see in the 30 gallon I have a moss dome, and they love going inside that.