Any tetra will work as a dither, but they also make excellent fry predators, and the more of them you have, the larger the toll they will take on your spawns. If you use tetras, don't do more than three.
An even better option, if you can find them are pencil fish (have small mouths not built for eating fry), or hatchet fish (top water fish, not built for grabbing things off the bottom).