Long story short, I have a 75g and 55g hooked up to a 20g sump (no, surprisingly they don't overflow if I shut both return pumps off), and neither of the tanks really have anything other than live rock in them at the moment. That part's a long story. But I've been trying to add a refugium to my system for forever. There's an assortment of things I think would be problematic with most typical solutions (clearance for a hang-on refugium, expensive/too much flow to replace sump with one with a refugium, difficult to plumb a second sump). So I was thinking: can I empty out the 55g, get a sheet of glass or acrylic, and partition off one side of the tank to use as a refugium? And I guess either drill holes through it so water can pass through, or raise it slightly above the water level and add a pump so water will gravity feed back into the tank. Not really sure how lighting would work, though. 21" seems kind of deep, and I have a 48" 2x54W T5 fixture that I can change the bulbs in but can't exactly cut in half. Thanks for any input. Didn't seem like a good idea to rush into this without getting some advice.