I had the deck screws left over from the big tank build.
I spent $67 on 15 2x3 8' studs and 2 4'x8' 3/4" plywood.
At the current state of the build, I've only used 10 of the studs.
The 24" tall tanks are 215 gallons and the 19" tall tanks are 170 gallons, so I end up with 770 gallons. Not as much as I first anticipated, but still substantial. I'm more focused on conserving materials and limiting waste.