Add Bleach
I use fresh water with a gallon of bleach. Even so, I have to pump the water out about twice per year. When I next do that I am probably going to add 2 gallons of bleach. Hot conditions in the Lake Ponchartrain area (latitude about 30 N) causes very foul odor. Was not so when the boat sailed just 300 miles further north. So, the need for bleach will depend on how warm your climate is. By the way my boat is a 240, which I assume has a somewhat smaller ballast tank than your 260.