Ideally, yes. But if you use RO
and are understocked and do frequent water changes you can get by without one. I didn't use a skimmer for about the first year I had my tank running. I had algae problems left and right. I got one about a year and half ago, and I don't know why I ever went without one.
If you plan on keeping corals/anemones I would reccomend one. I also know people (on this site) who have kept VERY successful tanks and have never run a skimmer on them.
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