One last idea/suggestion. Of course it may or may not be practical or have a big enough market. I was researching & thinking of installing a Two Little Fishies Phosban 150 Reactor, running the plastic bio-pellets, to fine tune Nitrates & Phosphates. I'm reading really good reviews on it and it seems like the one-stop solution and best way to go to address both issues. And apparantly the bacteria produced really makes the skimmer come alive and feeds many inverts like feather dusters, coco worms & other inverts - a nice additional benefit & selling point.
To install this, I THINK I have to go without the two cooling fans in the left-rear, and run it thru that opening (I'm trying to avoid doing a sump). It seems easy enough to do but I really would prefer internal components instead of hanging stuff off the back and having water splash on the walls as I'm servicing the unit. So this got me thinking how nice it would be to have a small unit that fit in that chamber some how. Since there's minimal water lift needed, I'm sure a tiny Rio-like pump would be all that's needed, or maybe it could be driven by the left chamber pump? The TLF says it can handle 150 gallons, so an RSM250-specific unit could perhaps be one-half the size of that unit. And this imaginary unit would need a flow adjustment valve to optimize pellet tumbling. I know there's not a lot of room back there, but if this could be combined with a rack....oh how sweet!
If the above is a non-starter, how about fabricating another "accessory device" for the fan opening? If it could fit two pairs of in-out tubes, that would double the fun! Folks could run multiple media reactors, a canister filter for some heavy duty water polishing and all kinds of things!
Best wishes for success with your RSM 250 products!!