There are many things one must be careful off when harvesting rainwater. Clean the terrace of the house before monsoon. Make sure no moss remains on the roofs. Also trim any branches that may be extending over the terrace. Otherwise the falling leaves by obstruct the pathway of the rainwater to be collected.
The storage tank must be sealed tight. Not even sunlight must pass through the seal. The water in the tank should be accessible either through a tap or a hand pump. Also cover the drainage pipe with a nylon net. Do not harvest water from the first 2-3 flushes of rain. This will ensure no pollutants pass into the tank. Also do not leave the tap open for extended periods of time. Seal any leakage immediately if detected.