Apparently, I'm still relying on a bit of mind reading. "Same volume" is referring to a comparison between 20 high and 20 long. Either is obviously twice the volume of a 10, but the 20 long has a bigger footprint than the 20 high. .a gallon is a measure of liquid volume. thus a 20 gallon tank has twice the volume of a 10 gallon tank. dimensions don't matter as long as they add up to the right volume(s).
i vote for RCS. they're hardy and breed in fresh water.