well from my experience.. I have drained and pulled down gas tanks from two vehicles, one is from a 1995 Nissan Vanette (gasoline) after 8years of usage and from our Assembled AUV (diesel) after 10years of usage, and to my surprise i didn't find any "latak" formation, sediments or anything of different in nature aside from the fuel!! its clean!! very clean inside... but what i have noticed is dirty FUEL filters that when agitated will release black, dirty fuel inside which indicates it needs replacement...

I suggest you change fuel filters and check your fuel pumps for functionality..