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It's not just fish, but what type of coral do you plan on keeping, if any?? The typical recommendation would be 1.5 to 2 times your water volume in skimming capability. So, if you're running a 125 without a sump, then a skimmer rated for 200 to 250g would most likely be sufficient..... though IMO, there is no such such thing as too much skimmer, especially if you're planning on any SPS corals.