There are several conditions in an ornamental or a farm pond that can cause obnoxious odors. Those odors can range from foul fishy smells to sewage smells or the rotten egg smell of sulfur dioxide. Odors can develop gradually or seemingly overnight and sometimes they can even disappear on their own, though not usually. Let’s start with that one, since it’s the most benign and usually self-solving. If you’re seeing frothy water along with the smell, you’re probably witnessing spawning activity. When fish spawn (usually in the spring), eggs and sperm are released along with other liquids, and in some cases, koi especially, that can produce a distinctly fishy smell. Fortunately, once the breeding cycle is over, the problem usually goes away without any action on your part. If you’re planning on a pondside backyard party, that may not be particularly reassuring, but individual koi generally only spawn for 2-3 days. Dozens of fish may spread out their 2-3 day schedule over several weeks, though. If supporting a new generation of fish is not high on your priority list this year, a couple of moderate water changes may lessen the issue long enough to enjoy your backyard bash.
Other causes of unpleasant smells usually point to poor water quality rising from any of a number of sources, and also indicate dangerous conditions for your fish. It’s critical to start your investigation by running a chemical analysis for pH, ammonia and nitrogen products, all of which can cause or exacerbate your problem. Start by addressing the chemistry so your fish are out of danger, and then work on rooting out the cause. Water treatments available at a good pond supply shop should perform well - check with an experienced staff member on how and how much to administer in your case. You might even bring a sample of your water in for them to analyze. Once your chemistry is within acceptable levels, spend a few minutes surveying your pond. Remove anything obvious like dead fish or animals and dead plants. Take note if you see evidence of uneaten fish food as well. Can you see the bottom of your pond? An abundance of sludge, or mulm in the bottom should be cleaned out and aeration added to disrupt low-oxygen zones in your pond. Anaerobic bacteria releases sulfur dioxide as it consumes this kind of waste, and is a prime source of offensive smells. Check your biological filter to see if it seems to be in good shape with a healthy colony of aerobic (beneficial) bacteria. If you’re not sure, you can always add a dose of bacteria from the pond shop. This good bacteria will process waste without producing dangerous toxins and unpleasant smells. In farm ponds without filters and pumps, you can compensate for poor oxygenation by adding an abundance of submerged plants along the shoreline. A fountain or waterfall can provide extra oxygen, and solar powered options are available.
