Skinny six spot goby

[Evilweevil]

So after about 6 months of having our six spot he has gotten really really skinny ,we have tried different foods and he does pick at them but he isn't getting what he needs , I am thinking that our sand maybe to coarse for him ,is there anyone in the red deer area with a fine sand bead that has room for another fish ? I would hate for him to die with out giving him a chance ...... All of our other fish are happy and fat n healthy never had ick or any probs ( knock on wood) just starvin Marvin ...... Any ideas?