This was posted by Peggie Hall years ago. I have it in my saved notes:
FRESH WATER MAINTENANCE
.25 cup bleach per 5 gallons
Fresh water system problems--foul odor or taste--are typically caused by allowing water to stagnate in the tank and especially the lines, creating the ideal environment for molds, fungi and bacteria that thrive in damp dark places. Here's the recommended method for recommissioning fresh water systems; this should be done at least annually:
Fill the water tank with a solution of 1 cup (8 oz) of household bleach per 10 gallon tank capacity. Turn on every faucet on the boat (including a deck wash if you have one), and allow the water to run until what's coming out smells strongly of bleach. Turn off the faucets, but leave the system pressurized so the solution remains in the lines.
Let stand overnight--at least 8 hours--but NO LONGER THAN 24 hours. Drain through every faucet on the boat (and if you haven't done this in a while, it's a good idea to remove any diffusion screens from the faucets, 'cuz what's likely to come out will clog them). Fill the tank again with fresh water only, drain again through every faucet on the boat, repeating till the water runs clean and smells and tastes clean.
Remember: cleaning out the tank addresses only the least of the problem...most of the problem occurs in the lines, so it's very important to leave the system pressurized while the bleach solution is in the tank to keep the solution in the lines too.
People have expressed concern about using this method to recommission aluminum tanks. While bleach (chlorine) IS corrosive, the effect of an annual or semi-annual "shock treatment" is negligible compared to the cumulative effect of holding chlorinated city water in the tank for years. Nevertheless, it's a good idea to mix the total amount of bleach in a few gallons of water before putting it into either a stainless or aluminum tank.
People have also expressed concern about the potential damage to rubber and neoprene water pump parts. Again-the cumulative effect of carrying chlorinated water is more damaging than the occasional "shock treatment." Occasionally, a water pump has been known to fail immediately after recommissioning, but it's always been an older pump in which the diaphragm and/or valve were already "on their last legs" and would have failed soon anyway. Far better to "push it over the edge" during spring recommissioning than have it fail a month later at sea with no replacement parts aboard.
To keep the water system cleaner longer, use your fresh water...keep water flowing through system. The molds, fungi, and bacteria only start to grow in hoses that aren't being used. Before filling the tank each time, always let the dock water run for at least 15 minutes first...the same critters that like the lines on your boat LOVE the dock supply line and your hose that sit in the warm sun, and you don't want to transfer water that's been sitting in the dock supply line to your boat's system. So let the water run long enough to flush out all the water that's been standing in them so that what goes into your boat is coming straight from the water main.
Finally, while the molds, fungi and bacteria in onboard water systems here in the US may not be pleasant, we're dealing only with aesthetics...water purity isn't an issue here--or in most developed nations...the water supply has already been purified (unless you're using well-water). However, when cruising out of the country, it's a good idea to know what you're putting in your tanks...and if you're in any doubt, boil all water that's to be drunk or used to wash dishes, and/or treat each tankful to purify. It's even more important in these areas to let the water run before putting it in the tank-wash the boat, whatever it takes...'cuz any harmful bacteria will REALLY proliferate in water hoses left sitting on the dock.