That foul odor or taste is typically caused by letting water stagnate in the tanks and lines, creating the ideal dark and damp environment for molds, fungi and bacteria. To flush the system, you have a few options.
Crowley’s Ship’s Store carries Puriclean, a biodegradable powder treatment that sanitizes the system and dissolving tabs keep it clean after every fill. Puriclean powder is $13.95 and the tabs are $11.25 at Crowley’s Ship’s Store. An alternate solution is one cup of household bleach per 10-gallon tank capacity.
Whatever your choice, follow these easy steps for cleaning your water tanks, do it annually, and you’re good to go. Fill the water tank with your chosen solution. Turn on every faucet on the boat, including the deck wash, and allow the water to run until it smells like bleach. Now turn off the faucets but leave the system pressurized so the solution remains in the lines.
Let this stand overnight for at least eight hours but no longer than 24. Then, drain every faucet on the boat. Remove the diffusion screens from the faucets so they don’t get plugged. 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. 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 solution is in the tank to keep the solution in the lines too.
If you have aluminum tanks, Puriclean is better than bleach because bleach is corrosive, but the effect of an annual or semi-annual cleaning with bleach is negligible compared to the cumulative effect of holding chlorinated city water in the tank for years. If you use bleach, it's a good idea to mix the total amount in a few gallons of water before putting it into either a stainless or aluminum tank. In the future, keep your water system cleaner longer by using your fresh water regularly. If you don’t keep water flowing through the system, molds, fungi, and bacteria will grow in your stagnant lines. Another tip: Before filling the tank each time, always let the dock water run for at least 15 minutes. The same critters that like the lines on your boat love the dock supply line and your hose, both of which sit in the warm sun.