The previous owner may have winterized the tank and used some kind of diluted anti-freeze, or the brownish stuff is just debris and god only knows what. I would try using regular house hold bleach mixture with water. let sit in the tank, and then flush it out with fresh water several times. If all you have is a foot pump (no mechanical pressure pump) you may want to see if you can access the top of the tank through a cap and wash clean the inside, and flush the water fill hose, and pump, and water lines. Thats my two cents.