is this to do with shedding?

[mim]

ok so oki had this first and now willow has aswell. oki had this problem ages ago and i was unsure of wot it was. he seemed to has a loss of skin on his back but it didnt seem to bother him and it healed but ever since then he seems to get this dark patch on his back. now willow has it aswell. is it to do with them shedding? they are still pretty young (only born last yr). im not sure at wot age they start to shed. iv never found any skin in the tank or water bowl and never seen them pullin off dead skin. any help would b great. the first pic is oki second is willow. cheers