DieselFusion

Why don't you drain that tank completely. Pull and inspection plate or sender and dry it out completely and look in there with a borescope or other camera to see whats going on? Drying it out completely will kill any algae you have. Then shopvac any remains and start fresh. I've seen numerous times on a few boats where they didn't put enough hardener in the resin and it was still sticky 20 years later.....maybe the fuel is slowly dissolving that?

As for Diesel Kleen, I;ve had great success with it. I managed/maintained a 1991 50' searay for 10 years and we had to change the racors after running 200-300 gallons of fuel through them for a while and they'd be packed with black algae each time to where you could only see half the depth of the pleats. The owner only ran that much fuel through them a year. I put the Clear tank diesel in the tanks a year ago. I just ran the boat 700 miles from Ft. Laud, FL to Myrtle Beach, SC this week, and ran 1500 gallons of fuel through her and didn't have to change the racors at all and they looked only slightly discolored.

I'm sorry, I did get a little off the line. My effort was to advise against a sales pitch, against adding any unknown product into your tank and disclaim that if it's on the web, it's true.

Then I got carried aweigh with my experiences of adding an other product to my boat.

I'm sorry my zeal distracted from the OP.
,rc