Yep, that's aiptasia - one of the most notorious pests in the hobby. The good news is there are several effective ways to deal with it. Here's a full rundown from easiest to most involved:
Chemical / DIY methods (spot treatment):
- Lemon juice - as mentioned, works well. Draw into a syringe, turn off flow, inject directly into the aiptasia. The acid kills it on contact.
- Kalkwasser paste - mix kalkwasser powder with just enough RO water to make a thick paste, inject/apply directly onto the aiptasia with a syringe. Very effective and reef-safe.
- Hydrogen peroxide (H2O2* - also effective as a spot injection with flow off. Use 3% concentration.
- AiptasiaX (Red Sea) - a commercial product specifically designed for this. Works very well and is easy to use.
- Joe's Juice - another popular commercial aiptasia killer, same concept.
Always turn off your pumps and flow first so the substance stays on the aiptasia and doesn't spread. Wait a few minutes before turning flow back on.
Biological / natural predators:
- Peppermint shrimp (Lysmata wurdemanni) - the classic biological solution. They will actively hunt and eat aiptasia. Make sure you get the right species - some look-alikes (like *L. boggessi*) won't eat aiptasia.
- Copperband butterflyfish - very effective but requires a larger tank and can be tricky to get feeding on prepared foods.
-Berghia nudibranch - specialized aiptasia predators, incredibly effective. They will eat ONLY aiptasia. Great for large infestations but they die off once the aiptasia is gone, so they're a temporary biological solution.
- Filefish (Matted filefish) - some people have success with these.
If you disturb or cut an aiptasia without killing it properly, it can release larvae and multiply rapidly. The key is killing it completely in one shot - which is why the syringe injection methods work best.