The numbers come from what are you trying to do. Low end is if sump just feeding a skimmer (logic being more than skimmer is rated is probably wasted flow) or on the high end if combining a refugium or using the return pump to provide flow for display.
I'm using my return to up the turnover of my BB in addition to a CL plus have a cheato refugium in my sump. Just using the sump tank size for the ratio I'm pushing about 13X/hour through it. Found with another pump 26x was too much as was getting micro bubbles in display and detritus wasn't settling out.
And if you go to Wetwebmedia, Fenner's a big Eheim promoter.
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