OK I am here!
I did use a Magnum as a return pump for my sump. It worked OK but as czcz stated, the flow rate is much reduced due to head height. In my case, the Magnum managed 150-200 gph
without any filters (empty media basket). Using the micron filter, flow rate is more like 150, but drops quickly as the filter gets clogged. After about a year of weekly servicing/cleaning of the Magnum, I gave up & now is using a hydor submersible pump (600 gph
rated, about 400 actual).
You might have better luck with the fluval. From what I read, it is a better canister than the Magnum & easier to maintain. <I might still be using the Magnum if it didn't take 1/2 hr or more to assemble & prime the thing every time I need to clean it.>