Ok. Have you seen any limpits?
Several different genus and many species make up the snail-like invertebrate family, commonly known as limpets. Limpets have an oval, laterally compressed shell that tapers to an off-centered blunt point. Limpets have a tiny hole at the crest of their shell where it becomes a point, which they use for waste and water exchange. Many of these species have a very large mantle that extends around and covers the shell. In fact, some limpets can be very beautiful with amazing colors, and make a welcome guest in an aquarium that does not contain SPS
The most common variety encountered in a reef aquarium is the Keyhole Limpet. They are a common import with live rock, and are typically colored in a mottled brown, black and tan pattern, and do not have a mantle that cover their shell. The Keyhole Limpets 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
As with many of the snail species, it is best to physically remove the limpet if you are unsure of its species and diet. Some limpet species are herbivores and are beneficial to even a reef aquarium. But 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 a sufficient control for the SPS
coral aquarist." - LA