Is the 20 gal an RDP Refugium growing Macroalgae? If so, I'd use that place for another zone of the ocean ecosystem, the Benthic Zone.
Darken that back overflow area of the tank with black plastic film and fill it with porous rock, not too small. Use LR selected specifically for it's live Sponge. Be sure to remove all filter sponge, and bioballs. Live sponge and other animals living in darker areas and under rock are an important part of the ocean ecosystem.
I have done this several times in different systems. Systems with Benthic Zone Refugiums are very stable and bring excellent health and accelerated growth of the animals in the main display. I have 4 or 5 types of Sponge growing in the Benthic Zone of one of my Refugiums in the Coral Farm. I could share some LR with Sponge with you.
Aloha,
Mark
