dirty fuel tanks

[derekh]

I have always praised fuel set. Other local boats had the bug but I didn't, or had I?. Last season the filters had to be changed twice. (Centrefugal type) I have removed the inspection plates on the tank tonight and the fuel seems clean but when drained, the floor of the tank has a black slime on it. even up the walls there is a black film. I have tried to clean it where I can reach but it is not easly removed. Can anyone point me in the right direction, which I hope is not remove tanks. Also what is the best additive?
Thanks for your help.
Derek

 

[Poignard]

I cleaned my diesel tank out using a hired steam wallpaper stripper. It took a couple of days before the water came out clean. I don't know how clean it was afterwards because there is no inspection plate but I got a lot of black muck out of it. I took the tank out of the boat to do it so that I could move it to diferent positions in the hope of getting all the dirt out.