Hello! Sorry your prized fish is not acting normal and you are loosing sleep :(
Not sure how long your tank has been running with your fish in it, or how you cycled it. What are your ammonia and nitrite levels? If the only thing elevated is the nitrates and ammonia and nitrites are zero that normally means that the tank has cycled and what you have now are the resulting high nitrates but this can take several weeks so if it is a fresh set up you have something else going on. Please check your water with an API Water Test kit for freshwater, it is much more accurate than test strips.
To try to help with your problem at hand, I would recommend a 50% water change and use Prime water conditioner according to the total volume of your tank. So, for a 55 gal tank you will use 1-1/2 cap fulls. This small amount extra of Prime is not going to hurt your fish and I would rather use a tad more just to be on the safe side. If you are not familiar with Prime, here is the website link. Seachem. Prime
Without seeing the marks on his fins, it's hard to say, but just recently we moved a 10" Wild Caught Oscar to another tank and he trashed around and got a skinned place on his side and the top of his head. Luckily we were placing him in a tank that was fully cycled so I researched and found 2 products that have totally cleared his damaged scales in a little over a week. They are
1. Acurel - BodyGuard RX - Vitamin Boost and Appetite Stimulant
2. Acurel - HealthGuard - Weekly Health Boost and Electrolyte Stabilizer
The medication above is a liquid drop and you use 1 tsp of each per 50 gal done on a weekly basis. So, if you are in the middle of doing lots of water changes for high nitrates, best to wait until this has calmed down, then start the medication.
Also, right after we moved this huge Oscar he would not eat. For about a week. I think he was just stressed from being moved. I would drop food in and net it right back out within 15-20 mins. With nitrates that high you don't want food sitting around in the tank, will just make it worse. You will need to stay up on cleaning the substrate and water changes, like every 2-3 days or so depending on how high your nitrates are. With the API test kit, nitrates in the orange (20 ppm and below) are tolerable, the more Red and higher the number goes the more dangerous it is to your fish and needs quick action. The clean fresh water will go a long way in helping the overall health of your fish and once your water is in good shape, he will start eating.
There is a product on the market called API NitraZorb that you place in your filter. Here's the link. We have used it before, it is rechargable and can be used several times over again. It helped, but the results were not immediate and not drastic. We just decided to keep doing what we were doing... consistent water changes, clean substrate, stop over feeding and the use of Prime water conditioner for the safety net that other water conditioners do not provide. http://www.apifishcare.com/Products/...x?ProductID=95