Phosphates – Invisible Troublemakers in Ponds and Aquariums

Green water. Nuisance algaes. Cyanobacteria. Poor coral growth. Random invertebrate death. All of these are problems found in freshwater aquarium, ponds and saltwater aquariums and leave many aquarists stumped. Most of them can be diagnosed with one simple water test however – the often-overlooked Phosphate test.

What is a Phosphate and Why Should I Be Concerned?

Phosphate is a naturally occurring compound with several sources in aquariums and ponds. The most common may be the source water used. Some municipalities and well water sources naturally contain phosphates. It can come from the soil in the area or from run-off into the water sources, especially in agricultural areas or areas that use a lot of fertilizer (phosphate is one of the main ingredients in fertilizers used on farms and backyards alike). While the levels may not be considered dangerous or high to humans, it can accumulate in aquariums. Phosphates can also enter an aquarium through the salt mixes used in saltwater aquariums, in the rocks and decorations used, and in the thawed water from commercial frozen foods.

The signs of high phosphate levels can be obvious (if you know what to look for), or might be a bit more subtle and hard to recognize. One of the most common signs that you need to check your phosphate level is the water color. If you see water in your aquarium or, more commonly, your outdoor pond that is pea green in color, test for phosphates! Some nuisance algae like hair algae or black hair or slime algae in freshwater tanks may also be caused by phosphates. Like we’ve mentioned, phosphate is a fertilizer. A fertilizer’s main purpose is to make plants grow. Algae is a plant, ergo phosphate will make it grow. If the lighting on the tank or pond isn’t excessive and promoting the algae growth, chances are its the water itself causing the bloom. Cyanobacteria, that notorious algae wannabe that plagues countess aquarists, is also given a boost by high phosphate levels in the system.

One of the more subtle signs of high phosphate levels is seen in marine and reef aquariums. In systems where the fish appear perfectly healthy but corals stop growing or invertebrates like snails and shrimp don’t seem to survive for long (especially after molting) may be high in phosphates. Phosphate appears to interfere with the invert’s ability to grow and harden its skeletal structure or shell.

Dealing With Phosphates

So, how do you get rid of it? Phosphate can be one of the trickiest levels to balance or eliminate. The first step is to identify the cause. Is the water you are adding to the tank already high? Try testing the water you are adding to the tank, straight out of the tap. In saltwater tanks, test a sample before and after mixing the salt into the water to see if it may be the salt mix you are using. Do you use frozen foods, and are you rinsing it before adding it to the tank? The water that frozen foods are frozen in sometimes has high phosphate concentrations and should not be added to the tank whenever possible – rinse it gently in a fine net before feeding. Are you in an area near a lot of farms, golf courses, or heavily fertilized lawns or gardens? If the phosphate is in a pond, are there fertilized flowerbeds around it? Run-off from rain may be washing phosphates from fertilizer into your pond or aquarium water source.

Once you’ve identified (and hopefully eliminated) where the phosphate is coming from, you can start getting rid of what you already have. Water changes with phosphate-free water can help cut down on your levels but can take time before you see a noticeable difference. In saltwater aquariums, protein skimmers can help remove wastes and decrease the phosphate levels, and refugiums can be used in freshwater and saltwater tanks. In refugiums, plants like Caulerpa in saltwater and Anacharis in freshwater use nutrients like phosphate and nitrate to grow and as you harvest and remove the plants, you remove those nutrients from the system. Refugiums are also a good way to grow fresh greens for the fish in your main tank!

Filter medias are probably the most common and effective ways to get rid of phosphates. Most of the common phosphate-removing media is “adsorbing” – meaning phosphate sticks to it like one of those sticky lint rollers instead of getting soaked up and absorbed like a sponge. When you remove the media, you then remove the phosphate. Some medias are rechargeable so you can “unstick” the phosphate from the media and use it again.

Phosphate can be tricky to diagnose and remove from a tank, but prevention and early detection can help you eliminate lots of problems down the road. I fought the battle with phosphate in a freshwater and saltwater tank myself (contaminated source water was to blame in my case) and can tell you that it isn’t impossible with a little persistence!