Well there is skinny (as in need more feeding), in which case high quality food, not necessarily bloodworms which are full of fat, will fill them up and then there is skinny as in stomachs sunken in so inverting. If its the second it is likely to be worms, maybe a parasite, or wasting disease hence my asking if you can post a photo. HFF sells praziquantel in liquid form. They are licensed to do so. Formalin is also an alternative however prazi is safer for you to use than formalin.