Love this idea. My first "tank" ever (I was 7) was a square glass vase. In fact, I actually went to Cost Plus and looked at their square glass vases

They have two sizes, both about 5.75" x 5.75, one is 12" tall the other 16". So the smaller water volume would be around 1 gallon, the larger a bit over 1.5 gallon (taking into consideration the width of the bottom and the fact that you can't fill it all the way). Should I go for the smaller or larger one? Thinking soil and sand, planted (val, java moss, guppy grass, etc.) and red cherries. What do you guys think?