1. Most come in off of liverock and then reproduce based on the amount of food they have. This is why tanks that are well balanced have a healthy amount of them and tanks that have too many nutrients have too many of them.
2. I think there are thousands of species of pods
, but all except the Isopod are beneficial as far as I know.
3. They help scavenge for debris and eat microorganisms and such.
4. I can see mine most when the lights are off or right after the lights come on. If you have moonlights, check near your sandbed at night and you might see some crawling on the glass.
5 and 6. I don't think the neccesarily contribute to anything besides eating some gunk. You may have too many so that it's unsightly but I am not aware of there being too many so that it's not healthy for your tank.
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