I'll throw my 2 cents in here. Based on the picture of the top frame, it is still fuctioning. The centre brace is still intact. I have a 90 in the basement that is in the same condition, that I am resealing. You can purchase new plastic frames from the tank manufacturer, if you desire. I have 4 tanks waiting to be resealed right now, and have built 5. If you don't trust the existing cross brace at the top, you could add a pice of 1/2" x 3" or 4" glass beneath it, using silicone to glue it to the tank.
This repair can be made sucessfully, by stripping the silicone out of the interior, using razor blades. When you are positive, that you have gotten all of it off, get a new razor blade and go over it again. It is important to remove all the old silicone, as new silicone doesn't stick well to cured silicone. The bead does not need to be as big as you want to apply, but I like to go just slightly beyond the edge of where the old silicone was, to ensure attaching to virgin glass. If you aren't used to sealing tanks, you can mask the eges of where you want the silicone to stop, using masking tape. This will give you a very clean edge to the new silicone. The arear to be sealed should be cleaned thouroughly with alcohol or acetone before applying the silicone. I usually clean the glass first with either windex or vinegar before that.
Don't use GE siliconeII for aquariums. You want an acetic acid type such as GE Silicone I, which has been used by many, for years.
With a tank that size you will probably need some helpers to roll the tank. While not essential, it will be easier to apply the silicone if you can position the tank for easy access. You want to be able to apply the silicone quickly, so that you can smooth the seams before they set up. The task may seem daunting, but with a little preperation can go very smoothly. Do a couple of dry runs, so you know exactly how you are going to do the application, and don't deviate. If you use masking tape, remove it carefully as soon as you smooth the seams. I use a finger or thumb to smotth the seams, but there are tools for the job. To me it is easier to wipe some build up off your fingers than from a tool. It is best to work in a well ventilated area as the fumes can be irritating.
As far as an angle iron frame goes, I would recommend against it. If you use steel, it will rust. In addition, structural angle has a radius in the corner, which won't allow the glass to fit snugly. It also is not necessary on the bottom, and will not replace the centre brace on the top.
If you want to support the bottom of the tank, use styrofoam between the top of the stand, and the bottom of the tank. It will take up any minor discrepecies in flatness in the top of the stand. If you were to use wood, or wood products, there is the risk of breaking the tank bottom if there was warpage or swelling. This is why tank manufacturers prefer to have the bottom unsupported, rather than risking the damge that warpage could cause. Personally, I firmly believe that a supported bottom makes for a much stroner assebly, but it needs to be done right.
This might seem to be a big job, and it is, but once done, will result in a tank that should give you many years of trouble free service. There is no reason, it won't be as good or better, than it was when new. I still have the first tank I ever built, 40 years ago, in use.The majority of my 20 tanks were bought as leakers. Hope this helps, and good luck. If you have any questions, pm me.
