what kind of media do you have in the filters?
Have you tested your tap water for high amounts of chlorine or ammonia?
Don't lose hope you can get through this, If the ray is eating that well you are doing a good job keeping him happy under the circumstances
First draws have filter floss for mechanical and the rest is a mix of ceramic rings and bio balls plus a couple of scrubbies to fill the tight spaces! My tap water has no ammonia and I am dechlorinating on every water change! The ray eats like a total champ he's constantly scouring for food and pounces on anything I put in the tank, however I think that's more to do with aquamans care than mine! How long can I expect him to live for if this ammonia continues to stay apparent? The last thing I'd ever want is for him to die on me!