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Senior Member
Aug 5, 2014
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President 43
How much bleach per 100 gal should I use to clean the water tanks and water lines.. how much would I use per 100 gal after the tanks and lines are flushed. thank you
I typically use a gallon to 100 gallons, let sit for 24-48 hours and then thoroughly flush.
I don't add any bleach inbetween sterilization and usually do it once a year.
I have fiberglass tanks and copper water lines, others with SS tanks may think it's excessive.
I have 2 x 60 gallon poly tanks. I had significant 'mold' inside the tank. You could see a scum line from outside of the tank. The water smelled horrible.

I used 1 cup of bleach for each 60 gallon tank. Filled the tank with water and let it sit for up to a week. Drained the tank and repeated the process. It took 3-4 cycles to completely clean the tanks and lines.
I have steel tanks, forgot to mention that
How much bleach per 100 gal should I use to clean the water tanks and water lines.. how much would I use per 100 gal after the tanks and lines are flushed. thank you

If you don't have a problem, I wouldn't use any bleach.

I wouldn't add bleach if you are filling the tanks from chlorinated city water.
most folks have a water softener and filter where I live, and the water in the tank has been there for over 2 years i have added more water but never drained it
When I dewinterize the boat I use about a cup of bleach for my 100 gallon tank. It's empty when I put it in. Then I open each faucet, hot and cold sides, one at a time until I can smell the bleach water coming out. That way I know I have bleach throughout my system.

I let that sit for 24 hours then run all the water out. Then I'll put some water (~20 gallons) in the tank and run it through all the faucets until there's no more bleach smell.

When I fill the tank I used an inline filter. It's probably not necessary but we have never had any foul taste or odor from the water.
If you're using city water, it probably will keep you tanks clean if turned over regularly. Long term storage would be better with dry tanks.
Bacteria can enter the tank in air thru the vent as the tank heats and cools everyday. So even sanitized water, but not treated, will become contaminated. I make all my water, and prior to the tanks the water goes thru a UV light sanitizer. Without treatment, my tanks will become contaminated. From there the plumbing can become contaminated.
I treat my water everytime I make a large batch. The boat holds 450 gallons and I usually let the tanks get down to a 100 gallons before making more. I use a cheap pool test kit, bought on Ebay for $5. I add enough chlorine to just show on the lowest point of the scale. I also use a set of filters before the galley sink that removes any chlorine and odd tastes (if any). Bar sink and ice makers are also on this line. Too much chlorine left in the lines can slowly ruin some rubber parts like valve, pump, and faucet seals.
It is actually easier to stay ahead of water problems than deal with contaminated tanks and plumbing.
What I do.

1/2 gal per 100 gallons to disinfect the system. Let sit for 3 hours but no more than 24. Rinse at least twice then fill up. After that, I will add 1 tsp bleach per 50 gallons of water when I add water to the tanks.

Not a recommendation, just a report.

Edit to add: Adding bleach to city water is likely unnecessary and I often won't if I know that I will be going through the water relatively quickly. But in the off season, we don't go through it as quickly so I will add the bleach to the system.
There are many suggestions on both RV sites and boating sites that discuss how to keep your system safe. Annual disinfecting and then regular servicing.

Funny how many flip out about oil changes and antifreeze changes... yet say....ho hum, just use city water and don't worry. Despite many boat MANUALS that advise otherwise.

This site cracks me up.... :) .....just proves to me what people are comfortable with or understand...which is most boating topics...when they don't .....it's quick, to the manuals everyone....

A quick search probably will show where the Headmistress has answered this many times for us.
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Do not use bleach if you have aluminum water tanks.
Big no no
What you do to [add to] the water in your tanks is greatly dependent on what water you fill with and what uses you use it for.

We keep our tanks clean by usually adding about 1/2 cup bleach to each of the 39.6 gal tanks every third fill up. We do not drink our tank water... it's generally agreed that water at our dock is not necessarily perfect!

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