Keyhole Limpet Snails will often start a debate among hobbyists. Some will tell you that they are fantastic algae eater, others will tell you they eat SPS corals and coraline algae. It all depends on the species of Limpet you find in your aquarium.
Some are good, some are bad. The problem with this is that it is very difficult to tell which are good and which are bad, even a seasoned hobbyist is unlikely to be able to tell them apart. Good or bad, they are all veracious eaters that will graze constantly!
They often enter your aquarium as hitchhikers on live rock, and are typically coloured in a mottled brown, black and tan pattern. They are typically half an inch long, or smaller, and feed on unwanted filamentous algae, cyanobacteria, diatoms and even hydroids. Unfortunately, in the SPS aquarium, they will also feed on the tissue of SPS corals.
How to control the numbers
As with many of the snail species, it is best to physically remove the limpet, especially if you are unsure if it is a reef safe one or not!
Some limpet species are herbivores and can be beneficial to a reef aquarium, but be careful, even the herbivores can become harmful if there is not enough food for them in the aquarium.
Limpets will typically not reproduce to any significant number in the aquarium, and physical removal is usually enough to keep their numbers significantly low.