What is this thing?

[brendanc23]

I recently noticed this thing growing in my tank when it was about half its size, but it now has grown pretty big and even has some more smaller versions of it sprouting up around it. I never specifically bought it and just randomly noticed it growing in my tank. Does anyone know what it may be?

 

[fish 321]

Aptasia a very anoying pest.

 

[coralbandit]

Aptasia a very anoying pest.

^^THIS^^
Remove them all in one piece ..
They reproduce through fragmentation so any pieces left mean new ones .
The Aiptasia anemones spread by fragmentation and can regenerate an entire individual from a single cell.

 

[Jesterrace]

I recently noticed this thing growing in my tank when it was about half its size, but it now has grown pretty big and even has some more smaller versions of it sprouting up around it. I never specifically bought it and just randomly noticed it growing in my tank. Does anyone know what it may be?

As mentioned aiptasia pest. When they are few and small Aiptasia X works well. They view it as food, not a threat and it effectively kills them from the inside out (safe for the tank though). I had a small one that came in on a coral frag about 3 years ago and I hit with Aiptasia X. Haven't seen any aiptasia since. Definitely much easier to get rid of before they spread.