Clorox, the bleach, is an easy way toward peace of mind with suspect drinking water. Quarter-teaspoon per 1.75 gallons, that's all.
I have read elsewhere that the Clorox needs to be spread through the water, not so easy to do in a closed 40-gallon sailboat tank. So in filling the tank with a hose, I would add Clorox every five gallons or so, rather than just all at once through the inspection port.
As an aside, my dentist has me using a tiny amount of Clorox in the Waterpik, just enough to taste it, based on recent studies at USC suggesting that that matches in effect the usual application of antibacterials by dentists.
Need to know how to disinfect and purify drinking water with bleach? Find out how much bleach to add to drinking water to safely sanitize it.
www.clorox.com