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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.

Potomac River green with Cyanobacteria bloomThe 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!


  1. avatar
    Paula Butterfield

    I have a small pond [L7′ x W3′ x D 1-3′]. It’s been established for years, with 6 good-sized [6-7″] goldfish. I’ve never had problems with cloudy water, disease, or anything else. Last October I had to have the house treated for termites, so I moved the fish to another pond prior to the treatment. I was assured that it’d be safe to return the fish after 3 weeks. I waited 6 months; they returned in April. Everything has been fine. I went out today to feed them, and the 2 large solid-gold fish were dead. The others were at the surface.

    I filled a 60-gallon plastic can with fresh water and moved 3 of the fish into the can. They seem to be doing ok. [I treated the water with API’s StressCoat.] The last fish floated to the surface of the pond about 2 hours after I found the first two.

    have a large honey locust that is shedding an unbelievable amount of seeds this season. They’ve covered the patio 1/2″ thick, and have gotten into the pond. I did a water test. Ammonia was between 0 and .25. Ph was a little over 7. Nitrite was 0. Phosphate was between 2 and 5, so I tested the phosphate in my tap water. It was about 1+.

    I’m sick about the loss of these fish; I’ve had them for years! I’m afraid to return the surviving 3 until I can get the water stabilized. Suggestions?? I appreciate anything you can offer!

  2. avatar

    Hi Paula. It isn’t likely that the termite treatment was the cause after so long, but it is possible that a large accumulation of the debris from the tree breaking down in the pond caused dissolved oxygen issues, in essence suffocating the fish. Was the pond cleaned thouroughly? I think with a proper cleaning you can get back on track, the chemistry doesn’t look too bad besides the phosphates. Is there any possibility the pond gets runoff in heavy storms?

  3. avatar

    What should a fish pond be as far as phosphates levels ?

  4. avatar

    Hello Gus, Phosphate levels should always ideally be at zero. It can be hard to reach or maintain that in an outdoor pond that can be exposed to runoff, especially near gardens or in areas with a lot of farms or agriculture, but levels should be as low as possible for the health of your fish and to control algae blooms. I would recommend trying to stay at least below 10-20ppm.

  5. avatar

    PHOSPHATE- A salt or ester of phosphoric acid,containg PO4-3

    If you don’t know what it is than look it up =)

  6. avatar

    Hello Alexis, You are correct. The Phosphate compounds found in aquariums are generally organic, making them esters and not salts, although some phosphates coming from more geological sources would be classified as salts. The differences and sources between the inorganic or organic forms could fill a textbook. For our purposes in ponds and aquariums, they can generally be viewed and treated in the same ways.

  7. avatar

    i has 7 small ponds on 7 acres of woods.leaves and dead trees everywhere.
    what is cheap way to lower phosphorus?
    i tried the copper sulfate.

  8. avatar

    Hello g, You would want to look for something to lower the compound Phosphate rather than the element Phosphorus. Copper sulfate really isn’t going to have any effect on Phosphate and can be very dangerous to the natural environment when used outdoors in a pond instead of in a contained environment like an aquarium. It should not be used in any system that may be exposed to a natural water source, watershed or groundwater. Phosphate in pond can be difficult to get rid of since it can be introduced through the runoff of fertilizers from farms and gardens and from natural water sources. There are filter medias that can be used in your pond filters to help absorb the phosphate and would need to be changed or recharged regularly. Three popular and effective once that are available in larger pond-appropriate quantities are SeaChem’s PhosGuard, Pura Phoslock and Two Little Fishies’ PhosBan.

    If you are looking for ways to control algae that may be a result of high phosphates, you can check out another one of our blog entries, “Algae Control in Ponds and Water Gardens“.

  9. avatar


    I have a 150gallon pond. 5, 4 inch goldfish. All of a sudden my pond get grenner by the day. I have tried bacteria and anti sludge non toxic solutions, freshwater snails, phosphorus control and it’s still super dark green/brown. Phosphate is really high. Took the plants out before treatment. Did a water change and everything. Maybe it’s the water I used. Its new so there isn’t My leaves in it Nd it’s raised off of the ground. The odd cherry blossom pedal lands in it. A racoon did get in the first night and it seems after that is when the dirty water look started. I have a badly bail in there. No filter though. Someone recommend a UV sterilizer. Looking for any advice. Oh! How long does it take for non toxic phosphate control to start working? Any suggestions?

  10. avatar

    Hi Tineke, I would say the Phosphate would be to blame for that green water. The time it takes for a Phosphate remover to work would depend on the Phosphate levels, the type of remover used and the concentration of the remover; the manufacturer of whatever you are using should be able to help you answer that question if you just check the label for contact information. A UV Sterilizer would help control the algae by breaking down the cells of the algae suspended in the water as it passes through the sterilizer. Barley Straw products also help break down the algae in the water but can take some time before they start having a visible affect; they can be used as an ongoing treatment in combination with the sterilizer moving forward. You can read more about pond algae control in our blog entry Algae Control In Ponds and Water Gardens.

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About Eileen Daub

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Marine Biologist/Aquatic Husbandry Manager I was one of those kids who said "I want to be a marine biologist when I grow up!"....except then I actually became one. After a brief time at the United States Coast Guard Academy, I graduated from Coastal Carolina University in Myrtle Beach, South Carolina in 2004. Since then, I've been a marine biologist at That Fish Place - That Pet Place, along with a Fish Room supervisor, copywriter, livestock inventory controller, livestock mail-order supervisor and other duties here and there. I also spent eight seasons as a professional actress with the Pennsylvania Renaissance Faire and in other local roles. If that isn't bad enough, I'm a proud Crazy Hockey Fan (go Flyers and go Hershey Bears!).