Q: I have a large water garden with goldfish and potted water lillies. In the summer, despite a biological filter and fountains for aeration, it gets floating clumps of hairy greenish-brown algae. I’ve tried adding algicide and cleaning the filter daily, to no avail. How can I get rid of this ugly stuff? Patti Genack, Beulah, Colo.

A: You’ll be glad to hear that you can stop fussing with the filter. Cleaning it too frequently (more than once a month) destroys the colonies of beneficial bacteria that filter the water. Forget algicides: they are toxic and don’t address the real problem in your pool. The overabundance of algae is fueled by a surplus of nutrients in the water — and the main source of that, I suspect, is the quantity of food you give your fish.     

Besides looking beautiful and eating mosquito larvae, fish snack on algae now and then. But when you toss them nutrient-rich fish food, you’re also casting fertilizer for algae on the water. Even if the fish devour every last morsel, the resultant manure is still an algae booster. Ideally, you should have only the number of fish that can support themselves on natural food in the pool. This doesn’t mean you have to stop feeding your fish, but you probably need to cut back considerably. Fish regulate their own numbers and growth rate to suit the amount of available food. You want your fish hungry enough to forage actively — which means the bigger ones will also eat fish eggs and hatchlings. For starters, don’t feed them more than once a day, and put out only as much food as the fish can polish off in five minutes. You might need to “weed out” some of the fish. The rule of thumb is no more than 1 inch of goldfish per 5 gallons of water.     

Avoid fertilizing water lilies as long as the algae problem persists, and take care as you tend the area surrounding your garden pool. If you feed container plants on the patio, for instance, and hose down the paving, soil nutrients may be washed into the pool. Even the soil in potted water plants is suspect, because of fertilizers in some commercial potting mixes. Grass clippings and fallen leaves are other sources of nutrients. Lay netting over the pool to catch and remove autumn leaves. Whenever organic matter collecting on the bottom gets to be about an inch thick, drain and clean the pool. Growing more aquatic plants can also help to inhibit algae. Like water lilies, algae is a sun lover — so try to leave very little open water where it can catch the rays. Water lilies and other desirable plants should cover two-thirds to three-quarters of the surface. That’s easily accomplished in midsummer; but early in the season, before the lilies really start to spread, algae has the run of the pool. As a spring stopgap, put in an extra measure of water lettuce (Pistia stratiotes), an attractive plant that spreads quickly and is easy to scoop out later, when the lilies have expanded. Water hyacinth (Eichhornia crassipes) also works well, but beware: It can invade public waterways and is illegal in some Southern states.     

Keep in mind that it’s perfectly normal for mosslike strands of algae to cling to the stems of water plants, as well as to the sides and bottom of a pool. Having some algae in the water is also normal. Healthy water will often be slightly green, though it should stay clear enough for you to see at least 6 to 8 inches below the surface. Scoop up clumps of algae as they appear. You’ll not only lift surplus nutrients from the water, but you’ll gather great material for your compost pile, too. If, after a month of reduced fish feeding and regular scooping, green water still shows no sign of clearing up, consider installing a UV sterilizer (listed in water gardening catalogs). By irradiating the water with ultraviolet energy, this device kills algae without harming the beneficial bacteria in your filter and on the pond’s sides.

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