You may get advice which is a lot better than mine (not hard to do), but if it were my boat then I would flush those fresh-water tanks several, several times. And add a little bit of bleach each time. Could very well be some buildup in there. Bleach should do it, I would think. You just want to kill the bacteria/algae. Personally, I do not drink water from the fresh-water tanks on my boat. I am sure it is fine, but just one boat dollar buys a heck of a lot of drinking water.