What is this weird sack type thing?

[Kully3xf]

there's like 5 of them on the underside of one rock... hasn't spread to new rocks yet; i assume i should take it out?
Any ideas?

 

[Salty B]

Pineapple sponge. Harmless. They come and go.

 

[Kully3xf]

Good news! Thanks! They look menacing!

 

[Salty B]

They like the dark/shaded areas so they dont become an eyesore. They're filter feeders so as little as they might help, we'll take all the help we can get.

 

[Kully3xf]

yeah, i just read that they are self limiting... they only feed on one thing; and it comes from non-ro water really... so my r/o membrane is getting weak, and i just ordered a new one; should be gone soon!

 

[little_fish]

Not true, they are in tanks run only RO/DI water as well

 

[Waddi]

I have tons on the side of my tank that had no lighting for months, the other side is completely clear. I use RODI.

 

[chichimom79]

I have them too and have always used RODI :D

 

[Bifferwine]

I had a lot of them in my overflows, and I only used RODI water. They can be found in any tank that has a food source for them.