what are some fast medium-sized freshwater fish?

 
What are some fast medium-sized freshwater fish?
i currently have a blue crayfish alone in a 15 gallon tank. i want to get some fish for the tank, and i heard that small, slow swimming fish will get eaten, but they get along with fast medium sized fish. well...maybe not get along with, but are too fast to get eaten. :P anyway, any suggestions?
6 zebra danios

Source(s):

me
stary
pinku
kij
you really need some top dwellers. ive got a 8" crayfish in with an assortment of cichilds. a 9" pleco and 3 yoyo loaches and the smallest is only 1". its never attempted to catch it or any of my other fish
I was going to say rainbowfish but in a 15 gallon tank it's going to be hard for anything medium sized to maneuver very much. Three dwarf neon rainbows might work.