Yes an external would require an additional pump (feed pump) to feed the skimmer consistently.....If your lucky and have the room you could try to gravity feed the skimmer without the use of an additional pump, but results could be different than using a dedicated pump.
I'm still trying to decide which feed pump I'm going to use for my skimmer. Mike from ReefSpecialty recommended a pump around 700gph, but the manual states 600gph, but I'm going with Mike's recommendations on feed rate.....worse case I'll have to dial it back a tad......As far as a good external sized skimmer....
XP 2000 external new $619.00 ReefSpecialty
*************** the same $619.00
Here's another option.....Cone skimmers are a little more expensive and still a good/quality skimmer, still utilizing the Bubbleblaster pump as well.....
This is the Super Reef Octopus 2000 external......it runs $534.95 at ReefSpecialty.......
Have you looked at the Diablo skimmers.......Another excellent fit for your system and with a foot print of 8.3" x 8.3" should be able to get the skimmer in the sump. What is your clearance as far as height inside the stand........ $295.00 at **************.....
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Reef Octopus® Diablo 6? In Sump Skimmer with Bubble Plate
Footprint: 8.3" x 8.3"
Height: 22"
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Diameter: 6.3"
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Rated up to 160gal.
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Optimal Sump Depth: 8-9.5"
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Modified cone neck design designed for easy transition of micro bubbles into cup
Powered by Italian made Sicce PSK-600 pump
Gate valves on the output for easier tuning of skimmer water level inside the chamber
2 year warranty on pump