If the fish are stressed, other problems can be more likely to pop up, so things like being bullied could potentially contribute. If I have a major problem in a tank, I'll do 2 50% water changes a day. As long as the temp in the tank stays pretty consistent and the water isn't chlorinated, and your filter is established, there's not really a way to do too many water changes. In some cases, some people have to do 90% water changes and the fish are fine. There's a saying in fishkeeping: We don't keep fish, we keep water. Basically, the healthier you keep your water, the healthier your fish will be; and in a tank, the only way to get rid of nasties is to dilute, dilute, dilute!