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I use a 7" cup and ditched the socks. I had 6 socks, 5 felt and 1 mesh, and would wash them when the last one was added to the sump.
Wife did not like cleaning them in the washer and they still look dirty. I did not want to keep replacing them so went with a cup.
I use poly fill and just toss...
This was posted by Marine Depot a few years ago. Hope it helps.
How to Choose a Properly Sized Return Pump for Your Saltwater Aquarium - Marine Depot Blog
I would think this has to do with trade regulations for one. Also, they would have to account for voltage and redesign of the plug for the US.
If a fairly new company may not be worth it but in time they could be available here if they get good reviews and company takes off.