Here is some info I received about cleaning water tanks,
Sm 376 Kimberlite
Although most people think only in terms of the tank, the plumbing is
actually the source of most foul water, because the molds, mildew, fungi and
bacteria which cause it thrive in damp dark places, not under water. Many
people-and even some boat manufacturers-believe that keeping the tanks empty
reduce the problem, but an empty water tank only provides another damp dark
home for those "critters."
There are all kinds of products sold that claim to keep onboard water fresh,
but all that's really necessary is an annual or in especially warm climates,
semi-annual recommissioning of the entire system-tank and plumbing. The
following recommendations conform to section 10.8 in the A-1 192 code
covering electrical, plumbing, and heating of recreational vehicles. The
solution is approved and recommended by competent health officials. It may
be used in a new system a used one that has not been used for a period of
time, or one that may have been contaminated. It's safe to use in tanks made
of any material:
Before beginning, turn off hot water heater at the breaker; do not turn it
on again until the entire recommissioning is complete.
Icemakers should be left running to allow cleaning out of the water feed
line; however the first two buckets of ice-the bucket generated during
recommissioning and the first bucketful afterward--should be discarded.
1. Prepare a chlorine solution using one gallon of water and 1/2 cup (4 oz)
Clorox or Purex household bleach (5% sodium Hypochlorine solution ). With
tank empty, pour chlorine solution into tank. Use one gallon of solution for
each 5 gallons of tank capacity.
2. Complete filling of tank with fresh water. Open each faucet and drain
cock until air has been released and the entire system is filled. Do not
turn off the pump; it must remain on to keep the system pressurized and the
solution in the lines
3. Allow to stand for at least three hours, but no longer than 24 hours.
4 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,
because 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.
5. To remove excess chlorine taste or odor which might remain, prepare a
solution of one quart white vinegar to five gallons water and allow this
solution to agitate in tank for several days by vehicle motion.
6. Drain tank again through every faucet, and flush the lines again by fill
the tank 1/4-1/2 full and again flushing with potable water.
People have expressed concern about using this method to recommissioning
aluminum tanks. While bleach (chlorine) IS corrosive, it's effects are are
cumulative. So 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 far more damaging over time than the occasional "shock
treatment." And it's that cumulative effect that makes it a VERY bad idea to
add a little bleach to each fill. Not only does it damage the system, but
unless you add enough to make your water taste and smell like a laundry,
it's not enough to do any good. Even if it were, any "purifying" properties
in chlorine evaporate within 24 hours, leaving behind only the corrosive
An annual or semi-annual recommissioning according to the above directions
is all that should be necessary to keep your water tasting and smelling as
good as anything that comes out of any faucet on land. If you need to
improve on that, install a water filter. Just remember that a filter is not
a substitute for cleaning out the system, and that filters require regular
inspection and cleaning or replacement.
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 certainly 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.