Glass anemone, Aiptasia
The best way to control Aiptasia is to avoid adding them to the tank in the first place. Carefully inspect any live rock for them before you purchase the rock for the anemones and if at all possible avoid purchasing rock with them on. Having said that I know it's not the easiest or most reliable method nor is it always feasible especially when ordering rock online or new rock that recently came into the LFS.
There aren't many natural predators for them and those predators are sometimes hard to come by. Treating a large tank will take time no matter what method you chose to use. Natural predators will take time to consume the entire population of Aiptasia and Chemical Methods takes a lot of patience and work.
The choice is really going to be with the person attempting to control the Aiptasia and the type of tank they have. This very large infestations, perhaps the best method of control is a combination of using Natural predators and Chemical warfare!
Peppermint shrimps, Lysmata wurdemanni
Peppermint Shrimp: Lysmata wurdemanni are often confused with a very similar looking non Aiptasia eating shrimp called the camel shrimp (Rhynchocienetes uritai). The major difference is the "hump" on the back of the camel shrimp is more pronounced then the peppermint shrimp. These are completely reef safe and wont harm your other corals within the tank. They are scavengers and once the Aiptasia is gone they'll accept regular food that falls to the bottom of the tank.
Butterfly fish: The Copperbanded Butterflyfish is probably the most common but most Butterfly's will eat Aiptasia. However, these are very delicate fish with a finicky diet and are hard to keep long term. They are polyp grazers, so aren't exactly the most reef friendly. Once the Aiptasia are gone from the system, they will look to any corals for an additional food source. They rarely eat frozen/prepared foods.
Nudibranches: As with most nudibranches they have a very specific diet, in that they will only eat Aiptasia or similarly flavoured anemones, so they are reef safe when it comes to other animals. The problem this creates is that once they have cleared all the Aiptasia from the tank they wont eat anything else and you'll have to add more Aiptasia to the tank in order for it to survive long term. This can be done with the help of either a species tank or refugium where you culture Aiptasia for the nudibranch.
Kalk: This can be applied a couple ways. The first (less effective way) is using a thick kalk paste to cover in any hole that the Aiptasia inhabits, the second is using a needle to inject the Aiptasia (harder method). This is effective when you can see the anemone and get to it before it retreats back into it's hole.
It takes a bit of practice and patience for this method to work.
How the additional Kalk added to the tank will depend on the amount added and the water volume of the tank. The more Kalk added the greater the chance of increasing the pH of the tank, smaller volumes of water will be affected sooner then larger volumes of water
Lemon Juice/Vinegar/Boiling water: Either one of these can be used to inject the Aiptasia with and will kill it just as effectively as injecting it with Kalk. The difference is you are injecting* an acid into the animal to kill it instead of an alkaline substance. It doesn't take very much to kill the Aiptasia so the addition of the acid shouldn't affect the water chemistry in the tank.
Stop Aiptasia: This is a chemical that is sold over the counter in the states; I'm not certain that it's available in Canada or the UK. I don't know much about it or how it's used or if it's reef safe, but I have heard good results from the chemical.
*Note: When using an injection method, use as fine of a pointed needle as possible so it's easier to penetrate the Aiptasia and it doesn't feel the invasion as quickly
Aiptasia, also commonly called glass or rock anemones, are nasty little creatures with a powerful sting. They are a common hitchhiker on live rock and when you first see them in your tank you can have an incredible impulse to rip them out of the tank! Unfortunately many people learn the hard way that this is a bad idea and even the smallest bit left over in the tank can regenerate into another Aiptasia creating a bigger problem then you started with.