Thank you everyone who has replied. I love having my tanks near (or on) my desk so that I can see them all the time while I'm at my desk, but the setup of my small room makes it hard for me to upgrade to bigger sizes. (2.5g on the nightstand, 5 gal fits perfectly on top of the shelf, 3.7g on the desk). Here are pictures of the tanks:
Fluval Spec V (5 gal)
Occupants: Betta splendens (male, veiltail), horned nerite snail, 1 amano shrimp
It's not as cloudy as it appears, I just had some issue with the lighting when I took the pictures. There are hiding places in the back corners, and behind the tree and driftwood.
Imagitarium Pro (3.7 gal)
Occupants: Betta splendens (female, plakat), red racer nerite snail
I have trouble getting hydrocotyle tripartita (and most plants) to grow well in my 5 gal; I love the way it grows off the driftwood in this tank. My betta is a female dragonscale plakat, and is able to swim around the driftwood on all sides.
It looks like a tight fit but there's a whole big pocket of space in the corner of the driftwood.
Fluval Spec III (2.6 gal)
Occupants: Dwarf shrimp (snowballs, mostly), horned nerite snail