Some fish thrive in softened water, while some others may prefer harder water. Knowing basic information about the water supply is essential to determining if or how much soft water should be used for aquarium water. Extremely hard water will contain high levels of Total Dissolved Solids (TDS), which may not be ideal for fish. Since water softening only exchanges sodium or potassium for hard water cations (calcium and magnesium), the TDS level does not change. Some soft water supplies are perfectly fine for fish, but soft water made from extremely hard water may need to be diluted with distilled, deionized or reverse osmosis water before it is acceptable. When beginning the changeover, note that any drastic change in the water supply could jeopardize "sensitive" fish. Partial water changes could be made at intervals using the softened water, but too much or too often may stress the fish. Please consult your local fish expert or veterinarian to determine if the raw and/or softened water supply in your area is appropriate for the kind of fish in your pond or aquarium, and whether additional variables need to be monitored.