Were you able to prep the area well? Sand it with 80 or 120 grit in order to give it something to bite to. Also clean with acetone once sanding is done. Then as said before a proper fillet and several layers of fiberglass cloth tape with each a little wider than the last should give you a good bond. Google how to do proper fiberglass repairs. Your picture doesn't look like there is any cloth just epoxy gunk maybe with a filler added?
I'd consider cutting another inspection hole or holes closer to where you need to work. If your current one is 2ft away you should have plenty of room to install another closer to this divider. If you intend to use the water tank for potable water you'll need to scrub and sanitize it once your repairs are complete anyways so having the extra inspection ports will help with that. It doesn't matter if its been 30 years since it was last used it still has plenty of ugly critters hanging out in there to make you sick. Read up on how to sanitize tanks properly, and things like biofilm. Simply treating with bleach is not a cure all. For that matter you might want to use a food safe epoxy as well if it's going to be submerged in your fresh water supply.
I'd consider cutting another inspection hole or holes closer to where you need to work. If your current one is 2ft away you should have plenty of room to install another closer to this divider. If you intend to use the water tank for potable water you'll need to scrub and sanitize it once your repairs are complete anyways so having the extra inspection ports will help with that. It doesn't matter if its been 30 years since it was last used it still has plenty of ugly critters hanging out in there to make you sick. Read up on how to sanitize tanks properly, and things like biofilm. Simply treating with bleach is not a cure all. For that matter you might want to use a food safe epoxy as well if it's going to be submerged in your fresh water supply.
