Floating waterlily leaves cover the water surface to provide shade for fish and reduce algae growth.

Lively, sparkling fountain or calm, glassy pool, water completes a garden and brings life to patios and decks. Water gardening has grown in popularity as people discover the tranquil beauty of water plants, colorful fish, and soothing sounds of falling water.

For more than four decades I’ve kept small water gardens and aquariums and shared my hobby with others. That includes coaching and reassuring new enthusiasts when the inevitable pea-soup-green water happens, the fish and plants die, or long strings of algae cover every submerged surface. They’re bewildered by the mess that appeared to be perfectly clear and healthy when they set it up a week or two ago. Looks are deceiving, however, because what’s going in the water—whether it’s a goldfish bowl or pond—is mostly invisible.

A complex web of bacteria, microorganisms, plants, algae and other creatures inhabit pond water. Nitrogen, oxygen, and light also play major roles. Their interactions affect water quality, which influences the pond’s appearance and the health of the fish and plants that live in it. Understanding this web of relationships it is the most important key to water gardening success.

Here’s how it works. As animals live and die, they release ammonia and other waste products. Bacteria and other microorganisms in the water process the ammonia and waste from animals and decaying plants, eventually turning them into harmless forms of nitrogen that plants can absorb. When the number of bacteria is sufficient to process all the available waste and the water contains enough plants to absorb the food, the water is considered “balanced”.


Splashing water adds oxygen that supports fish, plants, and beneficial bacteria.

Problems develop when any part of the cycle becomes out of balance. Here are a few of the most common scenarios and how to fix them:

  • Symptom: dying fish and plants. Cause: not enough bacteria and too much waste, which leads to a build up of toxic ammonia. Common in newly established and recently cleaned water gardens. Solutions: Change 1/3 of the water weekly, add fish gradually, increase the surface area available for bacteria to inhabit, increase the amount of oxygen by adding a fountain, or use ammonia-absorbing products in the filter.
  • Symptom: algae grows rapidly, covering surfaces or turning the water green. Cause: too much plant food and light and not enough plants. Solutions: add more submerged plants, decrease the light by adding floating plants or water-darkening dye, vacuum up plant debris and fish waste, manually remove string algae.

To learn more, read a how-to article on water gardening and Barley—the Secret to Clear Water. For products to help keep ponds clear, see the Pond Maintenance department.

-Ann Whitman, Horticulturist