We have never used a "water freshener"--and that involves boating for over 60 years and years of full time cruising and living aboard. Our tanks were from 300 to 500 gallons, but we also had a 60 gallon tank dedicated to a watermaker. Every 6 months, we do "shock" the water system--and never allow algae to grow in it (bacterial growth can occur--but is much rarer than algae). For shocking only you can use from 4 to 8 oz per 10 gallons of water. Let it sit in the tank no more than 4 hours--2 hours is enough. Be sure and run the chlorinated water thru all of the hoses and valves, including the hot water tank. Often the hot water tank is the source of problems. Then completely empty the chlorinated water out of the system. Fill and empty 3 times--then fill for use. There will still be some residual chlorine in the lines. You can use solids filter in the intake--but not a carbon filter (as above) which removes the chlorine. Never take water from a suspect source. If you are going to leave the boat for any time--then drain the tanks and hoses. For regular use, if necessary, one oz of household bleach per 200 gallons of water. (4 oz per 70 gallons is over kill). Be cautious about water in some remote areas, since chlorine does not kill cysts and protozoa like Giaridia.
We filter our drinking water after the tap, rather than attempting to carry plastic containers (remember that nasty plastic taste--you may be getting some material you don't want).
