Freshwater sounds simple, but it is actually the most complicated, and least understood aquatic environment by aquarists. Freshwater chemistry varies drastically from location to location. Geographical makeup of a location can affect water composition.
Rivers, streams, creeks, ponds, and lakes that are in areas where limestone is a major part of the topography tend to have high amounts of calcium, carbonate, and magnesium. The pH is usually well above neutral (7.0).
The Rio Negro in the Amazon Rainforest is an area surrounded by jungle that has a dense organic floor that helps make the water very soft, and a very low pH (4.0). In fact the water in parts of the Rio Negro are nearly as soft as distilled water.
Adding fish that are adapted to hard alkaline water to a soft water low pH system can often prove detrimental. Knowing what type of water chemistry your fish are adapted to is very important in becoming an accomplished aquarist.
Tap water, rivers, streams, creeks, lakes, and ponds always contain dissolved solids, that can include calcium, magnesium, carbonate, sodium, chloride, silica, lead, zinc, copper, iron, and many other elements. The major elements that we are most concerned with in freshwater for fish are calcium, magnesium, carbonate/bicarbonate, and sodium. All of the dissolved solids combined make up what is known as Total Dissolved Solids (TDS).
TDS is usually measured in parts per million (ppm). What that means is, if you have a TDS of 290 ppm in your tap water, 290 of the one million parts of water are dissolved solids. Pure freshwater has no hardness, no additives, and contains on only H2O and has a TDS of 0 ppm. Distilled water that can be bought at most grocery stores is as close to pure water that the average aquarist may ever want to work with in the hobby.