@AlexG Some ideas. Love your work so far. I think you could build the "ultimate QT system" based on what you did.
I've posted some comments on your vids, sub'ed and thumbed up a bunch.
A pic of that tube in the skimmer, how long deep it's going, is it capped only at one end? How much above the bubbler? So I can reproduce this, scaled down, with my skimmer. TIA
For the ultimate QT system - why not more plywood tanks with near airtight lids? Scaled way down, each QT tank with it's own independent filtration / built-in sump in the same "tank". Either 2/3 for fish 1/3 for sump or half-half. Then you stack them against a wall. You could have three or four high in a stack.
Make it look like it was part of the original design, add the "stack" on the left of the 720.
Mitigating cross contamination - if they are "sealed" with lids, air vents "in" and "out" being piping with some floss inside? I hated putting my QT for 2.5 months far away from my DT.
Thanks for the video comments on YouTube. The cylinder mod in the skimmer is capped on both ends and is a little longer than the skimmer neck which is 16" tall. Right where the bubbles are breaking the surface in the skimmer is where the cylinder rests. If that cylinder went into the water I feel it would create issues and potentially try to float. I will be releasing a video on cleaning this skimmer and I will show the complete mod without all the skimate.