Lighting

[apepper]

I think it would be difficult to do this while the tank is full because you will need room to maneuver the glass brace into place, plus the added risk if something goes wrong. If you leave the plastic brace in place while you install the glass brace(s) you probably don't have to worry too much about the added water pressure of a full full tank. I would think if you lowered the water level enough to have room to work you should be ok.

Two braces should be fine but you will be reducing your available access space for working in the tank, adding live rock, etc.