Well-lit water rich in nutrients encourages the growth of alage, in freshwater and saltwater fish tanks and in ponds. Algae are single-celled, microscopic organisms. Some algae types are free-floating cells, which turn water green. Other types grow as long strings, beards or tufts, or create rust-colored spots on aquarium glass. Most fish tanks and ponds contain some algae, but excessive growth looks unsightly and can be harmful to fish. When an algae population grows out of control, it's called an algal bloom.
Algae thrive in high light levels. Ponds exposed to full sunlight and containing few floating plants to shade the water often suffer from problems with algae. Algae can grow in waterfalls, where other plants struggle to survive.
Placing a freshwater or saltwater fish tank in direct light, or leaving aquarium lights on too long, also cause excessive algal growth.
Nutrients from uneaten fish food, fish waste, decaying plants and other debris feed algae. Excessive nutrients can cause algal blooms. Overfeeding fish in ponds or tanks, or overstocking with fish so that there is excessive waste, is a common cause of algae problems.
In ponds, additional nutrient sources include lawn fertilizers, contamination with animal waste and proximity to septic tanks. In fish tanks, another source of algae nutrients is tap water.
Warm temperatures provide good growing conditions for algae in tanks and ponds. Fish tanks for tropical fish require heaters to maintain the right conditions for the fish, but algae also thrive at warm temperatures.
In ponds suffering from algae problems, the algae often appears in spring, when the water warms up.
Alkalinity or Acidity
The alkalinity or acidity of a substance is called its pH level, and the pH level of water affects algae growth. In saltwater aquariums, water with a low pH favors the growth of diatom algae, which create spots on aquarium glass. Low pH water is acidic.