Stingray problem

Hi all. I presently have the only stingray I have ever kept. A motoro. He has never been a glutton like Ive seen on videos or on you tube. Ive had him about 10 weeks now and he lives in a 5 foot x 2 foot x 2foot bare bottom aquarium with a Silver Arowana. The Arowana is about 10 inches long and the rays is about 10 inches in diametre. Last night I watched him for quite a while and observed that he seems skinny. Pelvic bones showing and not looking very meaty or full. He is pretty active, not overly frantic in the tank and has experienced no bullying. Measured water parameters last night too. Temp- 29. Ammonia 0.01 on a nutrafin test kit after taking pH into consideration. Nitrites- 0, Nitrates between 10 and 20, pH-5.5. Foods been offered in the while I have had him, hikari sinking carnivore pellets, the tiny ones, Chopped prawn, hake, blood worms, snails from lfs and earthworms. Hes never shown any interest in any of these but I though he was eating when not disturbed by my prying eyes.
Any ideas on what I can do or change to try to get him to eat more. As far as I know he is from the east so Im sure he is captive bred???? They are never available here so info etc is very limited.
Any help will be very much appreciated. I would hate to lose this rare animal. Thank you.

Will try send pics asap. Aro eating well and growing nicely. 40 to 50% water changes every 3 to 4 days. I use prime and stability with every water change. Have a canister filter that does 1300litres per hour and a drip diy sump thing made with plastic drawers. Tank is very clean with no debris etc but will rinse out cansiter as possible ammonia source. pH has been the same last 6 months tank has been running. Will try send pics asap.

Snapped these 2 pics quickly. Will look at rinsing filter media, maybe wastes leeching ammonia if biological filtration limit exceeded.

last pic is of similar drip draw system and make shift refugium as ray tank has. Sorry for skew pic.