What in the world is this?

[justinl]

No many demospongia do not have any large obvious oscula (big holes). All sponges however have small pores called ostia... not always visible to the naked eye.

I don't think it's a chiton... it hasn't moved any and I don't see any of the shell plates. for the record chitons can be anywhere in the tank but usually graze on rocks.

you can scrape it off, but ten bucks says there's others in the tank you just haven't found yet.