Roofs and guttering are the main catchment area for household water tanks so its not surprising that substances such as heavy metals (from iron roofs, and paints), pesticides, decomposing leaf matter, bird and rodent carcasses and droppings, plus dust and grime can quickly make its way into your tank. This builds up over time to harmful levels.
When using roof water for drinking you must clean spouting regularly and keep your roof clear of bird and animal droppings, along with having your water tank inspected and/or serviced once a year. Healthy Water Tanks can help with all of this.