So I discovered a problem with my plan to use the 15g as a sump for the 20L: The base of the tank won't fit within the footprint of my stand. The 15g tank's dimensions are 16"x20"x14" (WxLxH). However, the width of the sump area of the stand is only 14", but 27" long, two inches short.
So... I have three options: (1) try to find a 20H to use as a sump; (2) do as I've done with the 29g and use a 10g; or (3) set up the sump outside of the stand. I'm not a big fan of 3, because this tank is in my office, and I'd like to keep things as streamlined as possible. 2 isn't great, because it's only 10g more water, and I've seen how quickly a 10g becomes crowded with equipment. 1 isn't spectacular, because I really don't want to fork of the cash for what the local stores charge for a new tank; so this option relies on finding one used in the area. Plus, I'm not sure how efficiently I could use the space of a 20H, given that it only provides an extra 4" in length and 2" of depth beyond a 10g.
If it's not one thing, it's another.
At this point, I'm leaning toward option 2 (10g), because I can readily pick one up for $10 or so. I'd be saving money over option 3, while only losing about 8" sq of space. It also keeps the streamlined appearance that I want for the office.
Thoughts? Suggestions? Random comments?