I just posted this on another link....but it may benefit you here as well....
There are a number of peppermint shrimp that look very similar that are sold with the name "peppermint shrimp,
Lysmata spp.. There is only one of these that is known to kill aiptasia and that is the
Lysmata wurdemanni. Again, my main point here is that since these many different peppermints are sold in the aquarium trade, you may not be getting
Lysmata wurdemanni, but a different
Lysmata shrimp. Here is a photo of the various
Lysmata shrimp sold in the aquarium trade.
Photo Reference
I've had three different species listed above before I came across the
Lysmata wurdemanni. Which is what I currently have and does attack aiptasia. I only use aiptasia-x on the large anemones and the shrimp loves to follow-up and tear them apart.
This is why you hear stories of peppermint shrimps not going after aiptasia. And another consideration is that if the shrimp is young or if the aiptasia is way too large, the shrimp will also not go after the aiptasia. These aiptasia anems can get 1.5-2 inches in diameter, the size of the shrimp.... this may be why they hesitate to go after the large ones..... again this has been my experience.
Other options
Aiptasia Removal - What's Your Preferred Method?
http://www.advancedaquarist.com/blog/aiptasia-removal-whats-your-preferred-method
The different methods devised over the years to fight back against encroaching Aiptasia:
* Commercial chemical products: Joe's Juice, Red Sea Aiptasia-X
* Electrical: Aiptasia Zapper
* Home-made: saturated kalkwasser paste, lemon juice
* Natural: Berghia verrucicornis nudibrachs, Peppermint Shrimp (Lysmata wurdemanni), Copperband Butterflyfish (Chelmon rostrata), Raccoon Butterflyfish (Chaetodon lunula), Long-nosed Butterflyfishes (Forcipiger spp.), etc.
* Un-natural: Fire (torch), boiling water (take precaution with un-natural ways - read article for more on this)
