The main function of dyes added to ponds or fountains is to give the water a more natural look. A blue dye added to the water creates a more attractive color. A practical function of dyes is to prevent the buildup of algae. The dye filters the sunlight passing through the water. Without the dye, sunlight penetrates to the bottom, allowing photosynthesis to occur and algae to grow.
Blue dye colors for ponds and fountains range from a light blue to a dark midnight blue. A blue lagoon color is available, as is a black dye. Dark colors and black are not recommended for ponds with any kind of life in them, such as fish or plants, because they block the light. Some manufacturers provide other decorative colors, such as red, yellow and emerald green.
According to Natural Environmental Systems, adding approved dyes not only gives fountain and pool water a natural look but also filters sunlight, is non-toxic and is compatible with most herbicides and algaecides. The dyes will not stain fish, birds or pets once added to the water and are safe for fish, plants, pets and people.
Some dyes have chemicals added to them that would have to be approved in your state. Check with your state to find out what is allowed. If you are adding other herbicides or algaecides, make sure the dye you are using is compatible with the chemicals.
When buying the dye either in powder or liquid form, make sure you buy the right amount for the size of your pond or fountain. Most manufacturers will show on the package or bottle how much water the dye will treat. Aerators or fountains will naturally pull the dye through the system, cycling it through the body of water. Instructions are usually included with the dye products.