Some parts of the world, including the UK have recorded pH values as low as 2.1, due to acid rain. Some fish keepers outside the UK have started to cover their ponds to protect them from acid rain. Rain picks up significant amounts of nitrogen, sulphur and sodium in the atmosphere. At this stage it is likely to be very soft. Without going into details at this stage, let me advise that you do not collect soft acidic rain for fish pond water changes.
Rock weathering Most of the inorganic substances in tapwater come from rock weather ing.
As the nature of the local rock varies, so does the content of the local water. The degree and rate of rock weathering depends on several factors. There are broadly speaking two types of rock of interest -igneous and sedimentary. Igneous rocks have been formed by volcanic action and are common in mountainous regions. They are extremely hard and dissolve slowly, helped by the slight acidity of naturally occurring rain.