The fish are actually called Satanoperca daemon, just FYI. They are very sensitive to poor water quality and don't seem to appreciate hard water with a high pH. What are your pH and hardness?
I'm assuming your nitrates are 25ppm? If so, that's a bit on the high side for long-term Satanoperca care. I wouldn't want them over 20, but they should really be 10 or below. These fish like very clean water, so frequent water changes are a must.
If it were me, I'd treat the fish with metronidazole in a quarantine tank for seven days. Metro is used to treat Hexamita, which is an intestinal flagellate that will overtake a stressed fish and cause it to stop eating. It's usually accompanied by long, stringy white feces. It's also been linked to HITH and some people claim a hexamita treatment is necessary to help fish with HITH. That's where I would start.
I use pure metro powder from Jehmco.com, 1/4 teaspoon per 10 gallons once a day for 7 days, doing a 50% water change each day before the new dose. Keep the temp in the mid to high 80s (84 - 88) during this treatment and increase aeration because warmer water holds less dissolved oxygen. Usually around day 5 or 6 the fish will start eating again, but I always follow through the full treatment.
You can extend the treatment if it appears to be working but the fish is not 100% back in terms of appetite. To entice it to eat, use something hard to resist like frozen bloodworms.