Little Help?

[IslandReefer]

Welcome Murminator,
I have a 15g tall...ie just 25% shorter than your 20g (22.4g). It has 2x 32w 50/50 PCs ,a powersweep (no filter)...for variable cross flow and an Aquaclear 500, modified as a refugium, that holds the heater,temp probe, more LR etc......all in all similar to yours, if you added the HOB refugium.
The aquaclear simply has a acrylic wall added to the main body to create an overflow...flow through the refugium into the tank...then deflected sideways by the powersweep....this creates good flow top to bottom and through the LR.
This Nano grows mushrooms and zoos like crazy! It also contains 2 candycanes (LPS),a pipeorgan coral, a brown plyotha, a scroll coral (soon to be moved), a blue ricordia,several xenia, two clusters of GSP, and three kinds of macroalgea. For critters it has 2 False Percs, a sixline Wrass, cleaner shrimp, apx 6 hermits, 2 turbo snails and several assorted snails.
Also I had 4 acros cooling their heels, on top untill they could go back into my other tank....lasted 3mths...went brown but lived.
That is, with good flow and regular water changes (I do 20% biweekly) your setup could support most low to mid-light demand corals (maybe upgrade/increase the lighting when you can) and critters to suit the tank size.
Goood Luck Murray....hope this helps