Fresh Water Storage on Boats: Keeping it Sweet


Fresh drinking water is essential on any boat.

It’s not unusual for boat owners with fixed fresh water systems to experience funny-tasting, foul smelling, discolored, or tainted fresh water systems. Fresh water stored on boats in fixed plastic or metal tanks can grow algae, mold, and bacteria in a single season—especially if the boat is in warmer climes and particularly if the water system is stagnant or used infrequently.

I’ve never owned a large boat with fixed fresh water tanks. My bigger boats happened to have removable bladder tanks, so when I accepted the job of managing a boatyard that catered to bigger boats and yachts I was due for an education on fixed water systems, their care, and specifically their cleaning and maintenance. The boat yard I managed is in the Northeast U.S., which necessitated water tanks to be emptied or winterized with antifreeze over the winter, so the problems were not as great as they might have been. But  over the years, hundreds of boats passed through the facility with drinking water issues.

Here’s how we dealt with them:

  • We added service-T’s whenever possible to all fresh water systems so they could be easily flushed and drained. A service-T is simply an inline piece of plastic where the T portion can be stopped off with a screw-on cap. The threads also allow a hose to be attached. This is a small one-time expense that pays dividends by having a conveniently located and effective drain and not having to break plumbing connections in future years to evacuate the system. In addition to providing a drainage point it can act as a place to add antifreeze or air for purging the system.water system diagram
  • Freshwater tanks and their connective pipes were treated, flushed, and drained annually.
  • If you have a hot-water heater in your system, turn off the power and drain this separately, but also flush a chlorine mixture through it (with the power remaining off). See the treatment notes below.
  • If you store the boat in an area that freezes and your boat’s fresh-water system has a motorized pump, it’s worthwhile to protect the pump by filling the piping system (not the tank) with RV antifreeze over the winter.
  • In spring, drain the antifreeze and flush with fresh water. A biodegradable product like Camco’s 40207 RV Spring Fresh will help to cleanse the system after storage.
  • Replace filters and screens. Filters come in two types, those used for filling the tank, and those used on the outlet side of faucets in your boat. Charcoal filters are relatively inexpensive ($20 to-$100 range) and are well worth the investment, particularly if you fill up frequently in locales of unknown water quality. Keep a designated white water hose exclusively for filling your tanks, and keep the ends screwed together when not in use to keep out debris.
  • If you do have an in-season incident of poor water quality, sanitize the system with chlorine bleach (see bel0w). A small amount of lemon or lime juice added to your clean fresh water tank can lengthen and maintain its fresh drinkable status due to the citric acid content.

To treat algae, bacteria, and mold problems, dose the tank with a chlorine bleach solution (1 part bleach to 160 parts water). For a 10-gallon tank, 1 cup of bleach is adequate—more is not better and will only cause you to flush more. Run the solution through all the faucets in the system until you smell the bleach at each faucet. This is important since the problem can exist in the plumbing as well as the tank.

Top off the tank and let stand overnight. It also helps if the boat moves around so the solution sloshes about, but in any case don’t leave the solution for more than 24-hours, to keep from damaging plumbing seals.

Flush the entire system with clean water the next day until the chlorine smell has disappeared (twice at a minimum), then drain the system. Remember to remove any screens or filters on the faucets prior to treating, as dislodged algae or scale can clog these.

If the tank has an access port, inspect the tank for algae, mineral deposits, or rust. In extreme cases power washing or scraping the inside of the tank may be necessary. Most often the simple bleach treatment works. If mineral deposits like calcium are noticed during inspection, treatment with white vinegar (just like cleaning your coffee maker) helps.

There are people who advocate their own prescriptions for cleaning fixed fresh water systems, such as hydrogen peroxide for metal tanks and muriatic acid for mineral deposit removal, but I think these are extreme and more costly. If maintenance of your fresh water system becomes an annual routine, the bleach solution, done in moderation, with a thorough rinsing, should work just fine.



  1. Rick Milner says:

    1/4 cup per 15 gallons sitting in all parts of the system for 3 hours before flushing is sufficient. Anything more could be harmful to seals, gaskets and plastic parts. This amount is FDA approved for water system sanitation and recommended by several plastic water tank manufacturers.

  2. Brett Bitner says:

    For those of you with stainless steel tanks, chlorine bleach can also be harmful if the concentration is too high (which the above recommendations are far from) or if it lingers in the tank for too long (which I feel 24 hours is too long unless you want to re-passivate the inside of your tank after this procedure…which could be reasonable for a particularly contaminated system). Rick is right in his previous comment; 3 hours should be plenty of time at the 1:64 concentration he recommends.