Re: Gas in water storage tank.
Chlorine is only good to sanitize the tanks. It only takes a small amount to do the trick. The bad thing is that the chlorine will also leach into the plastic tank as well. It still wouldn't hurt to try it since they are contaminated with the gas. The water probably be good to use for washing, but I'm unsure about using it to drink.
I bought my boat used and the fresh water tanks were pretty much done with. I went to the extent of cutting large holes in the top of them to gain access. Used chlorine, dish detergent, baking soda, and some other water treatments on the market. The water still came out stinking and unfit to drink.
During my transom/stringer replacement I replaced my tanks with six 4"X10' pieces of PVC pipe. This has given me about 42 gallons of fresh clean odor free water. With me installing them next to the hull below water line, it actually lets my boat sit about an inch more out of the water. Its been 2 years now and haven't had a problem.
I wouldn't use no more than a tablespoon per 5 gallons.....Good Luck.......SS