For some reason people tend to think that the return pump flow rate should not be more than the skimmer flow rate because the overflow water will bypass the skimmer otherwise. While this may hold true in a skimmer that is actually plumbed inline with an overflow it is certainly not the case for the typical in sump skimmer. The skimmer flow rate is completely independent from the return flow rate and even if the flow rates are matched there is no guarantee that the skimmer will cycle 100% of overflow water. In order to actually make this happen you would have to design the sump so the skimmer has to move the water from one chamber to the next meaning the flow rates would have to be perfectly matched which is virtually impossible. The other problem with this common assumption is once the tank has reached equilibrium the sump is simply an extension of the display tank, people tend to think that the water in the overflow box has more nutrients than the rest of the tank and it’s important that those extra nutrients don’t get put back in the display. This is a false assumption, skimmers remove dissolved organics and the concentration of these organics will be constant throughout the tank.
In my mind a higher flow rate insures the skimmer chamber in the sump also has a good turnover which will insure it always brings in new water but realistically once equilibrium is met the return flow rate will have no real effect in skimming efficiency. When deciding on a return pump it’s best to consider things that actually matter such as sump and display size, if powerheads will be used, micro-bubbles from skimmer and overflow, noise and your overflow drain limitations.
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