Q: We have two small ponds in our backyard. Every spring, I clean out the ponds and get the filters going, but I'm soon fighting algae in a battle that lasts all summer. I've tried adding fish and plants to control the algae. I've tried chemicals. But the algae always comes back. Is there any way to keep the water in these ponds clear?

—Laura Crain, Salem, IL

A: Roger Cook replies: Algae blooms occur when something in the pond — such as nutrient content or pH level, a measure of acidity — has gotten out of whack. Fish are a source of excess nutrients, as are decaying leaves, lawn-fertilizer runoff, and uneaten fish food. A good way to remove nutrients is to add plants such as water lilies, which not only help to restore a healthy balance to the pond but also add to its looks. Plants also shade the water, which reduces the sunlight that encourages algae to grow.

Check the water pH every six to eight weeks when the pond is healthy, but check it weekly if algae is getting out of control. A pH less than 7 is acidic; greater than 7 is alkaline. If the pH is veering out of the healthy range of 6 to 7, add appropriate chemicals to bring the pH into the healthy range; you can get them from pond-supply companies. Make sure the chemicals are labeled as being safe for fish.
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