I recently noticed some light tan stuff in my fresh water. Never seen it before in three years on this boat (1991 37.5) and seven on the previous one. I tried to flush the tank. Several rinsings and each time the initial quart of water each time is murky grey. I am hesitant to add bleach to an aluminum tank but the tank seems to be the source of the problem. Is the stuff more likely a biological one or a chemical reaction with the bleach (or something else) in the city water? The tank may have taken on a little salt water when the bow was under water in some heavy waves at some time so that may also be contributing to the problem. I suspect the tan stuff is biological and the grey stuff is chemical. Is there some biocide that I can use instead of bleach to kill the bugs? I am considering putting in an access cover to allow me to physically clean the tank or at least spray water into it to loosen up the crud. Do you have any other suggestions or observations about this problem that might help? Tanks for all of your help, Patrick