It looks like an "active" crack, one that still has ends that will keep growing until they reach another side of the glass or another crack. A patch should be as large as possible so the crack never extends beyond it. There will be a premium charged by your glass company for the curve the patch needs to match the bottom closely.
It is possible to score from the closest edge of the bottom up to the crack with a glass cutter then tap them together so the crack will be inactive. Even if the crack goes bigger than you want, at least it is done, and you can be secure your patch is big enough.
The patch should be at least as thick as the original glass. It should bridge across the tank for support.
It should be inside the tank. Never put a patch on the outside of a tank if more than 10" of water could ever be placed in it. A patch on the outside will make things worse, not better. It's like buttering the bottom of your toast. If common sense doesn't explain this to you, turn a pair of pants inside out, put them on, and then try to zip up.
To install the patch, support the cracked area from the bottom with a piece of plywood topped with a thin layer of styrofoam. Run Silicone I on top of the cracks, then press the patch on top and weight down with a 10 gallon tank you fill with water or the equivalent weight. Plastic pop bottles work fine and are easy to unstick if a little silicone leaks out. After the silicone has dried for a few days, run a second bead around the outside of the patch. There will be some air bubbles left under the patch, but doing this in two steps keeps them smaller and away from critical areas. Plus the silicone will only dry from its outside edges, so a patch will take much longer to set up if all done in one step.
