Did you or are you going to build a canopy?
No, I never did tackle that job. Too many other things to do... Maybe one day.
...does anyone know if the center brace going front-to-back on the top is necessary? Is it just to hold the frame steady? No tank weigh directly falls on that.
It's been four years since I built the stand, so I don't remember a lot of the thoughts that went into it. But I'll give it my best shot...
If I remember correctly, I was looking for a stand that had more lateral support (as opposed to vertical support). Many of the commercial stands that were sold in the big box stores (PetsMart, Petco, etc) were built with only four (4) legs (one in each corner). I was about to purchase one of them, but when I took a hold of them and tried to move them, they had a lot of lateral movement (which was kind of scary considering that it was going to hold a tank that weighed about 400 lbs). Since I had small children at the time, I could imagine one of them grabbing the stand, pulling on it, and the whole tank coming down on them. So I decided to do a "Tim the Toolman" number and build my own stand. So the middle braces are not there for vertical support, but are added mostly for additional lateral support.
In addition, if you want to have two doors on the front, then there needs to be something in the middle (in between the doors). So this was also a way to add a center post for the doors.
Hope that helps.