Echo brings up the problems of the oft-quoted "inch per gallon" guideline.
On that note, I 100% stand by my earlier statement and recommendation. Reason? I've done it before, and seen others do it before (my brother, for instance, keeps 3 rasboras in a 2 gallon tank filtered only by live plants and very frequent water changes). My and my brother's fish are doing fine, have been for 4 plus months. Now, I know someone will pick that one apart, so let me preempt for you... how do you know a fish is doing "fine?" My answer: colorful, active, alert, and feeding. I've seen harlequins in 40 gallon plus tanks, and I've seen them in tiny systems. Honestly, the only difference is the difference in the school, not the behavior.
Any other conclusion would be humanizing fish, which is a fallacy.
The motto of this fish forum is diversity. A diversity of successful fish keeping experience. My opinions and experience may go against the grain, but they are just as good as any other opinion because I care about my fish and do my best for them. I do not agree with the majority of the parrot-talk, that I call it, within the hobby, concerning how "useless" ten gallon and smaller tanks are, but that is merely my opinion.
We all have opinions. And opinions are not fact.
Diference between fact and opinion?
Fact: You cannot keep a healthy, growing oscar in a ten gallon system.
Opinion: Five gallon tanks are inadequate for anything except bettas and snails. They are cruel and inhumane.
To hijack even more... has anyone played Super Mario 64? There's a mini-level in there where Mario is swimming around in a 2-liter bottle, approximately the size of a juvenile endler. That's what I thought about when I read Echo's post, how much swimming room is available.