I'd please

[fosser]

Found these today while cleaning my fluval 304 canister. These are all over the inside and are hard to remove.

 

[fosser]

 

[saxman]

Pineapple sponges (Scypha sp.). They're cryptic filter-feeders.

 

[fosser]

Are they safe? Should I try and remove them or let them build up?

 

[gemmy]

They are harmless.