Retic stopped eating

I have a 2.5 year old Retic Stingray that stopped eating within the last couple of weeks. (I just had shoulder surgery, and have been laid up recovering so I'm not sure exactly how long it's been). Before that he was a big healthy eater, but now he shows no interest in the food. He is in a 200 gallon tank, and the water parameters for ammonia, nitrate and nitirite all test in the lowest section of the readings. I put in prazy pro, two doses over the past couple of weeks, and still no change. I'm trying to change the water more often as well. He was eating shrimp, and tilapia, and some pellets, but now nothing.
Any advice on anything else to think of.
I do have the tank seperated with a smaller female in a small section of the tank. I was trying to grow her out a bit more (she is 4-5 inches, he is 8-9 inches) before I put them together. I did put her in and he was chasing her around pretty bad, trying to bite her disc. I did have a couple of motoro's in with him for 2 years until they got too big for my tank and I sold them. Could it be that he is just lonely, and I should try putting them together?
Any advice is much appreciated.
Thanks

I have tried everything I can think of. Shrimp, Tilapia, nightcrawlers, silversides, and pellets. Can't find a place with blackworms in town.
I use the liquid test kits, and they are always in the zero. He's not super skinny yet.
I took out the divider, and he chased the female around for a bit, but now they're friends. I thought it might spark him eating.
I seem to remember something similar happening at the same last year. Maybe it's something in the city's water around this time?
Pup was added at least 4 months ago.