First, deep breath! These are unlikely to kill your fish, certainly at this size.
There are many ways to remove them.
1) If the rocks they are on are small, you can remove the rocks and use a bic lighter to burn them off. That'll get them! Fairly easy if the rocks are removable, and very effective.
2) You can also inject with lemon juice, kalk paste, aiptasia X, boiling water, etc. Be aware that unless the *entire* anemone is completely dead, it will grow back. Any living tissue at all will grow back.
3) Try peppermint shrimp. These are hit or miss. I've had good luck with them, but I hear a lot of these guys ignore the aiptasia in favor of easier food. Also, fish or shrimp that eat them carry the same proviso as #2. They rarely get every last bit. They control aiptasia rather than eradicate it. When the shrimp is gone, the aiptasia returns. Not a problem if you like the shrimp, though.
4) Berghia Nudibranch. Obligate aiptasia eaters. WILL eat it. Will NOT eat anything else. Hard to find, but very effective. They will devour every last cell of aiptasia and not touch anything else. But they can be pricey and hard to find. When the infestation is gone, best to give them to someone else. Really not cost effective except for major infestations.
Hope this helps show the options!
