Controlling biological growth in rain water tanks is one of the most important aspects of rain water harvesting if the water is to be used for human consumption. There are a few ways of doing this, the two main methods are dosing a sanitizing agent or circulating the water through a UV light. The problem with dosing is that you need to remove what you added, for example chlorine before the water is consumed. This is where a UV light system is nice; it leaves no residual traces in the water. Ideally you want at least two tanks; one for the rain water to top up and another that is kept clean and constantly circulating through a UV system. The first tank should feed the second via sediment filtration and a UV system; this will give you a tank of usable water with little need for more treatment.