the issue is the tank is full, if the box goes all the way down to the bottom I will recommend RTV108 but, if it's just a "floating" style box it's going to be very hard either way to fix without emptying the tank. again, either way you're going to have to consider emptying it low enough to be able to reseal it. we are talking 300 gallons of water and that's no small amount if it ends up on the floor.