I've had luck with this:
I've also used Aiptasia Rx from blue line with some success but f-aiptasia is more forgiving since it basically forms a shell around the aiptasia and works great in tight spaces.
Frank's Tanks F-Aiptasia
If you've ever had aiptasia anemones in your prized aquarium, you'd certainly understand how this product got it's name. Aiptasia anemones can kill your corals and even small fish. They are capable of stinging and stressing even larger specimens. So why not just kill them??? F-Aiptasia Killer...
clearchoiceaquatics.com
I've also used Aiptasia Rx from blue line with some success but f-aiptasia is more forgiving since it basically forms a shell around the aiptasia and works great in tight spaces.