Sidelite has it right! Also they make tabs to put in the tank each time you fill it. I always add those and keep using the water in the tank up....or just drain it and refill every couple of weeks to keep it fresh.
I would say to try bleachin the tank. Was discussed here earlier in the season. Somethin like 1 part bleach to 4 parts water. Let is sit for a day maybe then flush the tank a few times then use the water purifying stuff after. We use the aquatabs. They are little pills and each is good for 5 gallons. Always gives the water a bit of a chlorine smell that lets ya know its clean.