Cleaning fresh water tank with bleach

There are a lot of variables. First of all, as one has said above, if you have mold in the tank, then it was more than likely in other places. Being a certified pool operator, you want to have a high enough chlorine content to kill the mold and other water bound organisms. Chlorine strength varies widely in household products. In this case, I would put the whole gallon of bleach in the tank and fill it. Then pump it through out all of the system. Both hot and cold. Let sit for at least eight hours, then flush until the chlorine smell is gone. Some will say this is overkill, but do a search on water bound illnesses and you can see some of the stuff that can wind up in your water.