I think it depends on the person, on the fish, and on the tank. In general, I think 2.5 is acceptable, but if you have the money and space you really should get a 5 gallon minimum.
I currently have a fish in a 5 gallon, and a baby fish in a 2 gallon. When I look at the 2 gallon, I can't imagine using it as a permanent home, it's so tiny! I also have a 2.5 gallon hex tank that I'm planning on fixing and setting up, and it looks a LOT larger than 2.5 gallons to me. I'm thinking of using it as a shrimp tank, but I might end up putting a fish in it.
That's where I fall on the scale. :) If someone has a tiny tank, I'll suggest going with at least 2.5 gallons, but I'm not going to keep pushing it if someone doesn't want to listen. It's just not worth the worry and stress, and it's not my tank or fish.
Last edited by Flyby Stardancer; 07-18-2013 at 08:14 PM.