I've got 5 tanks and only one of them is what I would consider "perfectly level". The others seem to be off a tiny bit and the bubble on the level is always between the black bars in the middle, but may be barely touching one of the bars every now and then. Also, the tanks are all more level front to back than they are side to side. Sometimes when I fill a tank I will notice that the water line reaches the bottom of the top plastic frame on one side of the tank before it reaches the other. It is not a big difference but it is noticeable. I have checked some of my floors but carpet doesn't lend itself to measuring a level floor. Another thing I have wondered about is how level are the plastic frames that are placed on tanks? Since they are glued directly onto the glass, isn't there room for a little unevenness here too?