Hey everyone, I've almost filled my aquarium rack, now I have:

On the top shelf terrariums, jarrariums and live food cultures

On the 2nd shelf from the top, starting from the left: 1.5 gal opae ula volcano shrimp jar, 6gal paludarium (probably I'll add some vampire crabs in a month or so) and 4 gal (2.5 display+1.5 sump) reef tank (with a pompom crab and some sexy shrimp)

On the 3rd shelf I have a 10 gal chili rasbora/sunkist noecaridina tank, and a 7 gal blue dream neocaridina/amano shrimp tank

On the bottom shelf there's only an 8 gal shallow tank with five pygmy corydoras, neocaridina shrimp culls and a hara jerdoni.

Now, I'd love to add something in the remaining space, and an old 3.5 gal I have laying around would fit perfectly, what would you build with it? I'm not in a hurry or on a particularly tight budget, so give me some creative ideas

(I'm mostly into inverts, but I don't want to make another neocaridina tank)