lol @ replacing the whole tank. Some people.
you have several options. Unfortunatly all require you to drain the setup to fix it.
1. it looks to be on the 18"x21" side pane, that's not that expensive to replace
2. bracing, you already have the plastic one, but a metal or wood bracing around the bottom of the tank will hold it in place. You still will need to put some epoxy in that crack first but the bracing will give you piece of mind
3. epoxy the crack and then add more silicone on the inside. Due to the nature of the crack, you're kind gambling here as it's reducing the strength of the tank in the spot that has the most pressure.
the first is the safest (while still being sane about it seriously it's 3/4" crack on one pane, not an 8 foot one spanning all panes, goodness) but its only the safest if you're comfortable doing it yourself. If you're unsure of your abilities, find a professional who can repair the tank for you. That goes for all options actually.
...wait there's one more option
4. take the panel off (carefully) epoxy the crack and then flip it up-side-down when you put it back on, this will put the crack on the non-pressurized top rather than the bottom.