My stingray will keep curling his Disc after he eats, at first I thought this was because of the fish that I had in the tank with him (discus) so I removed the discus encase they was pecking or scaring him and put them in a separate tank, however he remained doing the curling of the disc. He has been doing this for couple of weeks now And never done it before. Had him since august. for the life of me I can’t seem to work out why he keeps doing it. He will swim like normal and window surf the glass, but it seem every time I feed him no matter if he only eats a couple pieces of food or finished the hole lot that i put in he will then go off and curl his disc. He has different food which are earth worms, shrimp, pellets, muscles, prawns. I do have another stingray but she is in another tank as I don’t want to add her and stress him out why he is curling his disc and I don’t know why. So he currently by him self. Is this normal behaviour that he picked up or is it the death curl please can someone advise me as I’m going out of my mind, tank temp is 28c ammonia is 0 no2 is 0, only has that fake rock in the tank as it was brought by my little boy the rest of the tank has nothing and is bare bottomed 
