my week old blenny died in a cocoon?

[im_a_clownfish]

Hi All,

I went to the lfs and got myself a bicolor blenny that had been in the store for 3 weeks. He was doing great in my tank, but after a week I woke up one morning to find him dead, wrapped in a whitish looking membrane, looking a lot like a butterfly cocoon. My nitrates/nitrites/ammonia are at zero in the hospital tank, water temp is at 78, salt gravity is normal, ph is normal.
Any idea what this could be? A bacterial infection of some sort? Im surprized the fish lasted 3 weeks in the store if he did have something. Maybe bacteria from my hospital tank? Its strange he went that quick. And even stranger is the cocoon. When I took it off of him the body looked fine with no signs of parasites or other bacterial activity....

Any ideas???

Thanks!

[PetMax]

its just fungus that grows on them after they die. Its something like ich, its always present in the tank.