Don't rely just on treating the fuel if you know there is already a sludge residue. Get at it and clean it right out. Use Marine 'Diesel Marendo' in any fuel used in the future. This will keep your fuel in tip top condition but also keep your injector pump and injectors clean and fully functional.
Check inside your fuel tank visually though the inspection hatch every service and you'll not go far wrong. If you find this difficult, many marine mechanics have an inspection camera/scope you might use.
http://www.wessexchemicalfactors.co.uk/product-detail.php?id=21
Try my local supplier. The write up explains the benefits.