To put it simply,
1). get a piece of acrylic the tanks thickness that extends the crack 3-4 inches all around. If its to the wall of the tank, its fine. Just make sure you put the patch up to the tanks wall when you weld it on.
2). weld it as mentioned with weldon #40
3). (optional) you can flip the tank over put the weldon on the crack then sand it down so the crack will be "filled" just for extra reassurance.
The part you patched will be stronger than any part of the tank so you don't need to even second guess the structural integrity of your fix. Those are great dimensions btw, I would DEFINITELY fix it.
Just a quick note for you, if you do fix it then sand it down as it looks like someone tried to incorrectly patch it with a patch that's too small. Weldon will "melt" the two individual pieces (tank and patch) into one, this is why that particular patch will be stronger than anywhere else on the tank. If it was me, I would just get a panel that will cover half of the bottom of the tank and weld it on the half of the tank where its cracked. You'll have to cut the pieces and play a little tetris fitting it together but its well worth it. And I would also remove the old crappy looking foam and replace it with a very straight piece of compressed wood at least 1/2inch thick. Top acrylic manufactures know foam is pretty useless on a correctly built stand and actually glue the wood to the bottom of the tank.