What is this guy
 

So lately my tank has been sprouting with alot of these guys that look like brown anenomes. Just wanted to ask you guys what they are and if they do harm to my tank.
http://i571.photobucket.com/albums/s...f/IMG_3228.jpg
http://i571.photobucket.com/albums/s...f/IMG_3223.jpg

I may be wrong but that looks like Aiptasia and if they are aiptasia they must be removed. If we positively ID it there are people here who can advise on removal. My CUC has a peppermint shrimp (two more coming my way from a friend who has too many). I got them simply because I love my cleaner shrimp and wanted to add as many different shrimp and inverts that I safely can as I find them as interesting as the fish. Peppermint shrimp are a natural way to control aiptasia though if you have an outbreak you will need either a large crew of them or will have to opt for manual removal. Like everything else in this hobby peppermint shrimp eating aiptasia can be hit or miss.

thats what it is. if it isnt removed it will start to spread and it can sting touching corals. i dont suggest going out and buying livestock to fix a problem where other methods prove to work better.

You can't control an existing problem by throwing a live stock solution at it. However, I like peppermint shrimp and will have 3 in my tank. While they are hit and miss when it comes to controlling aiptasia I would hope that a couple in a tank with no aiptasia could control an outbreak if one happens to be introduced.

Once you have an infestation you need manual removal.

However I am just a SW noob and I defer to people with your experience.

I know of an individual with a 180 gallon reef tank that allowed for a major outbreak of aiptasia (200 plus count).

He indicated that he purchased 25 peppermint shrimp (he works for a LFS)...and they cleared almost all Aiptasia with about 25 remaining.

Things to be learned from above story:
- Aiptasia WILL SPREAD QUICKLY if not taken care of.
- He indicated he would have been better off manually removing, but you can imagine the task. to remove you need to literally "chisel" some of the rock off, otherwise the roots are left there and aiptasia will just regrow.

My advice, (also a noob) - if your live rock in which it is growing will not cause you to restructure and break down whole strucutre, manually remove it. For others, get some peppermint shrimp and enjoy their beautiful coloration and hope they do their job.

Again I am very new to the forum and learning all the does and don'ts on this, so pardon my silly questions.

Hearing you don't want them in your tank, how do you prevent them from growing in the first place? what are signs to watch for when they start growing? it looks like the one in the photo is quite large already, how long did it take for it to grow that big?

Thanks a lot :-D

That mushroom is beautiful! Is it that purple, or was the camera generous that day?

Aiptasia is hard to find until it is in your tank. It is not a huge problem if it is caught early. I used peppermint shrimp with great success, they actually ate a small outbreak for me. I am one of the lucky ones though. I have heard of success with squirting lemon juice directly into their open mouths, or the same trick with boiling hot water.

They often close up or are on the underside of rocks you buy, so they get easily transported from the LFS. I personally keep a few peppermints in my tank just in case...