Inside of White Freshwater Hose Is Black

[bene505]

I disconnected the vent hose off one of my fresh water tanks and looked in the hose. Even though the hose is white, the inside of the hose is black. Does anyone else have this situation? I'm wondering if it's mold and whether I need to replace all my hoses. I'll bring some Q-tips to the boat on Sunday and see if I can learn more. She's a 2000 Beneteau 50.

In a related quesiton, what do people use to really cleanout the inside of their tanks? She's on the hard with ample water from a nearby hose, so I can easily rinse the tanks out, so putting bleach or something in is not a problem. Should I fill them up and put some On&Off in (weak concentration) and let it sit for a few days? (I know, probably not a good idea, but I thought I'd ask.) The tanks look to be fiberglass with stainless fittings.

[camaraderie]

Yes...it is mold and alge Bene. Replace the hoses. use clear, reinforced water hose.
Clean the tanks with a bleach shock treatment and then maintain them with a little bleach each time you fill them.

[Minggat]

If you have a watermaker, be careful not to get chlorinated water into the membrane when you do a fresh water flush.