There are a couple of 20 High sizes out there One of them is 16" tall (tank only) and the other is 17" tall. We have a 17" tall unit right now. The factory top and hood that uses floresent bulb is 3" tall. So right at 20" over all. Of course I got a glass versa top on it now with a dual bulb T5NO light set up and its a bit more low profile. I think its right at 18.5" tall with that on it.
Keep in mind you have to have about 12" above the rim of the tank for cleaning, otherwise it is awkward. If you don't have 28" or more, regardless of lighting choice, it will be a pain in the butt.