Thanks, Steph. I bought the shelving at Lowe's (make sure to select a heavy duty option that can hold a lot of weight per shelf).
https://www.lowes.com/pd/Style-Selections-18-in-D-x-47-7-in-W-x-72-in-H-6-Tier-Steel-Freestanding-Shelving-Unit/999990412
My original plan was to put a 20 long on the aquarium/middle shelf and 3 nanos on the plant shelf, so I wanted to make sure it could support the weight. I think this one holds an estimated 600lbs per shelf - but I won't be testing that limit! If I intended to fill the entire rack with aquariums I'd have bought Edsal shelving.
I then bought 1/2" plywood, which I got the lumber yard worker to cut for me since we are renters and own limited tools. I hand-sawed the notches so the wood fits around the corner posts (this could be done nicer with power tools we don't own, but it got the job done). By cutting notches in the corners the wood covers the entire top rather than a strip between the posts. I then sanded/stained/polyurethaned the wood. Stain is varathane dark walnut. This took some time, but the final look turned out how I imagined. It is a bit difficult to make the shelves level.
Overall, a great shelter-at-home project that really transformed my office nook. 🙂