Rainwater can be a satisfying zero-cost source of naturally soft water free of chlorine and its derivatives. There are, however, some caveats to consider before you decide to rely on the weather to keep your pond healthy and happy. For ponds located near major highways, industrial plants, large cities plagued by smog, and even farms, you’ll need to take into account pollutants that may be carried into your pond along with the rain, for example. Lead, organic particles, acidic and even radioactive substances may have been picked up by raindrops as they fall through the atmosphere. In significant rain events, runoff from your own yard can carry with it small (or large) amounts of fertilizer, weed killers, and biological material from compost.
To keep your pond and fish healthy It’s a good idea to regularly test rainfall in your area for impurities and pH level, whether or not you live in an area associated with acid rain. Once you know your local rainwater makeup, the EPA recommends that you filter and disinfect it to ensure it is beneficial to your fish. Situating your decorative pond upslope or creating barriers with impermeable liners to divert runoff during heavy rain will also defend your labor of love from debris and local pollutants. While large, natural ponds are more likely to tolerate the effects of rainfall, regular testing could alert you to dangerous conditions before disaster strikes. Seasonal events like wildfires in the western US can turn normally clear air into an ashladen cloud that’s hard even for people to breathe - a pond that collects and concentrates that level of pollutants is likely to be deadly to both fish and plants. In extreme cases like this, you probably want to consider some more direct action to protect your pond, such as a lightweight temporary cover that can be easily deployed.
