Bleach (chlorine) does damage...
Aluminum, rubber, neoprene, and flexible PVC hoses (never heard that it damages plastics, though). But you'll also notice that we recommend using it to recommission fresh water systems--an apparent contradiction. An annual or semi-annual recommisioning is also gonna do SOME damage...but just carrying municipal water--which is chlorinated---all the time actually does more damage than the occasional "shock treatment." Life in general is a series of trade-offs...and if adding bleach to each fill actually accomplished anything, the fact that it cuts the life expectancy of the rubber and neoprene parts in a pump in half , or reduces the normal 20+ year lifespan of an aluminum water tank to somewhere between 12 and 15 years would be worth it. But it DOESN'T "purify" the water in the tank...ALL it does is add a chlorine taste while it accelerates the demise of the system parts. Otoh, the annual or semi-annual recommissioning might take a year or two off the lifespan of the parts, but it DOES clean out the system. There have been a few cases in which water pumps died immediately following recommissioning...but in every case the pump was old and the rubber or neoprene parts that failed would have failed within a short time anyway. I consider their failure due to recommissioning somewhat of a blessing...'cuz it's better to have 'em fail then. while you're in port and able to fix it, than two months later on the 3rd day of a week long cruise...with your mother-in-law--who isn't sure she likes boats anyway--along...and you have to try to explain to HER why she has to brush her teeth with club soda.
