Silver Dollars, Tiger barbs or most Tetras.
Depends on what you want them to achieve. Some little school fish do very well in larger predator tanks, the shoal representing a fish too lrge to be readily eaten.
Do you want dither fish. High speed movement. Or food?
I've always liked Silver Dollars they seems to have some personality too which is always a plus. All 7 that I have like to follow me as I walk across the tank. Seems they know what time I feed them because if I'm a little late in feeding them they are all at the top of the tank looking for the food to drop.
Silver dollars are great, i would get those for sure. They are active, I know they are common but I wouldn't care. You could do silver dollars as well.