In a 5 gallon the water parameters are too unstable to maintain except for those that truly know what they’re doing. I’m assuming you’ll be doing the water changes but a 10 gallon will offer more stability so it won’t ruin everything if a water change gets delayed.
If you absolutely have to go with a 5 gallon then I’d get a betta or have a colony of cherry shrimp. Both are good choices.
In a 10 gallon you have a few more options such as schooling fish that aren’t so tiny that she’ll lose interest in them. Ember tetras aren’t bad. Pygmy cories could inhabit the bottom. Perhaps clown killifish? Hmm... it’s rather hard finding fish for a tank this small but I’ll get back to you once I find more.
Is there anyway you could bump it up to a 20 high or a 29?
Thats really not that true. You could just use a gallon jug for your weekly water changes. I find smaller tanks to be a lot easier than larger ones.
Youu could keep a betta, cherry shrimp, amano shrimp, kuhli loaches, and a mystery snail all together or you could keep one pea puffer by itself. You could also keep fish like dwarf gouramis, white cloud minnows, celestial pearl danios, neon tetras and almost any schooling fish from petco or petsmart that stays under about 2 inches or larger, slower fish that dont get much bigger than 3 inches.
The total length of all of the fish full grown should not exceed about 12 combined inches. If you put an airstone in you could push that to 18 combined inches. The more fish you put in the more often you will need to do water changes. Live plants will help to alleviate problems with the water chemistry.