Maybe he doesn’t like the food anymore? Maybe try a different food to see if he will eat it

Could be a change in water quality and or doesn’t like the food try something else

What do you feed them

I feed them with Tetracolor flakes and JBL bits. I'll buy Sera and see if he eats, feed him a couple months ago with it

Sorry for the gramatical error, english is not my first language. Edited with the correction

They supposed to be in the tank, not on them

i'm no expert but im rather sure they're meant to be in the fish tank and not "on" ;)