+1 on the Dwarf Puffers, but they need lots and lots and lots of plants or decor to break the line of sight between each puffer. They get quite aggresive with each other, so with all the live plants you'll break the line of sight. They do best in tanks that have diffused lighting so get lots of floating plants and they also do best with little to no current, so baffle your filter. What I suggest, if you're interested in these guys is that you set up one of those tanks, and do a NPT, let the tank go through a silent cycle. Also, don't try to remove any pest snails and let them overrun the tank. It'll be free food for the puffers. After the cycle is done, introduce the puffers to their new home, they'll thrive.Have you looked into dwarf puffers? ADORABLE. You could do 5ish in a 10 gallon. They need a somewhat densely planted tank, and pond snails.. They need to eat snail shells regularly to keep their "beaks" trimmed.
Well that's why, once I decide what species I want, I'm going to research the particular species, just like I do with every pet I get.FYI, not all fish in that list are advisable for tanks under 20g. Many of those fish are schooling fish, some needing larger schools that require more space. Many of those fish are very active fish, needing more swimming space than a 20g (or less) allows.
For example, many people think that because Neon Tetra are small that they can be kept in small spaces. This is not true, they need more swimming space than one might think and should not be kept in anything smaller than a 20g tank. Or, as another example, many people list a 20g tank as the minimum for a group of Bloodfin Tetra. But after having kept them myself and seeing their full size at maturity (more than 2", almost 2.5") and seeing just how active they are, I would never recommend they be kept in another less that a 3' tank, at bare minimum. Even more, Red Eye Tetra (Lamp Eye Tetra) get almost 3" and are just as active. They should never be kept in a tank under 3', preferably a 4' tank.
(You can click on the shaded names to read this site's recommendations on tank size.)
So... Just FYI. :-D