For a tank that size, you'll need 1/2" thick glass. Measure the inside dimensions then cut a piece of wood that size. Make sure it fits just barely, snug, but not where you have to force it. Sand/trim if necessary. Take it to your glass shop and have them cut your glass the same size. I suggest doing this because it's more accurate than a measurement and you have something there that you know fits to compare it to. Ask for some 2"x2" or thereabouts scrap pieces of glass while you're there. You'll need 4 for each brace.
Make sure they grind the sharp edges on all 4 sides of each piece. If they don't, you'll likely cut yourself on it which isn't fun at all.
Now that I'm thinking about it, my 225 had the braces on each end, not the middle one.
The scrap pieces of glass are going to be used to hold the brace up while the silicone cures.
Set the glass brace in place then make a small mark with a sharpie on the inside of the tank where the bottom of the brace is. Put the brace off to the side as you won't need it for a couple days.
Put a glob of silicone on each scrap of glass, line it up with your mark and squish it to the side of the tank on the inside. Tape a piece of masking tap across it to hold it in place while it cures. Don't push the scrap completely against the glass as you'll want to be able to get a knife in there to cut it free once the braces have cured. Repeat for each side.
Let these cure for two days to be sure they'll support the braces. What it does is provide a little support for the brace since 1/2" glass is very heavy.
If you don't want to use the scrap glass method, you can build something out of wood to support the brace while it cures or stack a bunch of phone books under it.
The brace is the easy part. Put it in place, run a bead of silicone along the edge on top and bottom, then smooth it with one stroke of a plastic spoon, your finger, or whatever you prefer. Just don't go over it more than once because that'll make the seal look funky.