A protein skimmer will help to remove organics before they have a chance to breakdown into ammonia then nitrite then nitrates. It will also help to aerate the water increasing oxygen and driving out CO2.
That sadi 20 ppm is not a bad level of nitrate, especially in a fish only tank. Of course if you have a reef tank you want to continuously drive that number towards zero. Your fish being stressed is definitely a cause for concern. Makes them more susceptible to the parasite if it is exists in the tank. However if the parasite is in the tank you have to take steps to removekill it off. This is done by leaving the tank fallow for at least 6 weeks, 8 is better. That will interrupt the life cycle of the parasite and cause it's erradication. Of course then to ensure you don't introduce it again, anything with the exception of your own newly mixed saltwater, must be quarantined for that 6 to 8 week period.
