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They have a very powerful sting. So, if you ever plan on having corals (and other stuff you don't want stung) they're not so great. Because, as mentioned, they multiply like weeds, and for your corals it'll be like living in a mine field.
if kept in a seperate rufugium style tank down flow of your main tank, they can actually be quit helpful in cleaning the water. theyll sting and grab everything that floats by making the water crystal clean. its nothing that other filters can't do, but its a good natural way of doing it. just thought i'd share