I have a 200 gallon 23 year old fiberglass fresh water tank. Located in the engine room. Blisters have started to form on the interior walls along with some black spots. The access (from the top) is limited, ie not in all sections, because of baffles. Ideas on how to clean or should it be replaced? Either way I suspect I’d need to pull up the floor of the salon for adequate access to the top.