I would replace it. The frames do alot more than people think. It's a structural element if the tank is designed to utilize plastic trim. The bottom is of the utmost importance since it bears the entire load of the tank... especially if the corners are completely separated by cracks. The main load bearing element is the silicone between the panes of glass, but the trim does play a part in reducing the bowing of the viewing panes and stiffening the corners. Some tank manufacturers use the trim as a jig to build the tank so it's usually a very tight fit.
The issue with cracked corners on the bottom is that a sudden lateral impact could cause the trim to shift under the weight of the tank. Silicone does not adhere well to plastic. Its unlikely but possible. Replacement trim isn't expensive. If you don't want to spend the cash to replace it you could at the very least use a 2 part epoxy "plastic weld" and some bar clamps on the corners.
I've got a couple small tanks (40b) sitting on cracked trim. Im biting my nails trying to plan for replacing them both. Its a risk I wouldn't take on a larger tank.
I paid like $16 a trim for some 40b tanks at the lfs. I'm a diy guy but it's hars to fabricate anything for less than $32
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