Help, discus developed a dent in his flank

[crayonchan]

Hey guys,

I've been feeding my discus freeze dried black worms, peas and spirulina pellets. I feed 2-3 times a day, until his belly is fat.

Recently, seemingly over a day or two, my discus developed this dent in his flank. He eats well, and is active and visible, behaving very normal.

Has anyone had experience with this, and knows if this is something I should be concerned about? Am I underfeeding my fish?

Thanks!