If you are looking at rain water harvesting it is important to decide what the water will be used for. For example if you just want to use the water in the garden then you can just use some pre tank filtration and a storage tank. However if you are looking at using the water in the house it is very important to control the level of sediment and biological activity in the water, again depending on the use this will differ. If the water is not for cooking and drinking just removing dust and other particles from the water with sediment filtration after the tank should be sufficient. If the water is to be used for cooking and drinking then the level of biological activity in the tank needs to be kept low and depending on the setup selected for treating the tank water an additional point of use filter might be needed.