As in many things, the best way to deal with an overgrowth of algae is to prevent it in the first place. Fortunately, most of the tools you’ll use in an aquaponics system to keep algae from getting out of hand will also serve to bring it back under control. Except in the most unusual circumstances, you’ll be dealing with green algae, which survives by photosynthesis, just like plants. It can be single-celled or multi-celled, but it generally floats freely within the water and can give it a cloudy green cast. Aside from looking unappetizing, algae overgrowth can clog up pumps, pipes, and other equipment, and a large bloom of algae can lead to deadly imbalances in your water chemistry which may lead to fish death and crop damage. Algae is pretty ubiquitous, and it’s likely that you have some in your system already, but the key is to keep it from getting too comfortable.
Like any plant, algae depends on the availability of nutrients and sunlight, and these are the major keys to controlling it. Generally, you’ll see algae overgrowth in your fish tanks rather than in your grow beds, because your crops should be consuming all available nutrients before algae can get a running start. In fish tanks, nutrients are widely available as nitrifying bacteria do their work, and algae can jump in first on the feast before that water is circulated around your plants, so that’s an ideal place to concentrate. If that’s what you’re seeing, the best and quickest way to resolve things is to deprive the algae of light. That can be as simple as arranging some tarps over the tanks to provide dense shade - some light is ok, but bright sunlight is asking for trouble. If you do have trouble with algae in your grow media, you may be able to add an extra layer of gravel above the water level that will block light from getting down to the algae - the details will depend on your setup, but you get the idea. For preventing future overgrowth, keep in mind that algae, just like your crops, thrive on food, but you don’t want to have so much food hanging around that you permit unwelcome guests. Keep an eye on nitrate levels in your water and either remove fish or add plants to make sure that the system is creating just enough nutrients to feed the plants - not too much and not too little. Without lots of leftovers, algae won’t find anywhere to get a foothold.
