Hey,
Ok, I'm probably going to get ripped on for telling you this, but I've had moderate success helping turtles....uh, "get revenge." When I was younger and saw two turtles not gettin along, (especially if one bullied another to the point the victim always swam away if it saw the bully coming or went to hide)I would occasionally hold the bullied turtle and manually make it chase the bully! Sometimes, the bullied turtle would instinctively open its mouth and bite in aggression so the bully turtle would become afraid whenever it saw the former victim. I repeated this process couple of times and often the turtles stopped fighting...weird. When I was young, I liked to experiment more.
Hahhaha...this was probably not the best advice or method to treat this problem, but hey it worked for me and many of the turtles are doing fine now, together or alone. Don't risk it however; because it might be stressful for the turtles. Instead, watch them closely and perhaps provide more hiding spots. Sometimes, the behavior subsides once the bullied turtle gets used to its new home. If you want to skimp on the tank costs, you can always substitute by getting a large, cheap rubbermaid container. =P
Johnny