I am stumped, high phosphate no algae...


New member
Maybe you can help me figure out why my phosphates are so high. I measured w/ Hanna phosphate checker and Hanna photometer . They both read 1.8 yeah that's right 1.8.

RO water coming into tank is measured twice with both instruments at 0.0. Salt mix is reefcrystals .

I feed pellets flakes Daily, mysis three times weekly rinsed with ro water.

I am running biopellets in a reactor . Now going on two months .

I have absolutely no algae problems what so ever . I grow bonsai and two other SpS with no problems . My z's and p's , LPS , and RBTA' show no signs of stress either .

Nitrates are less than 5 ppm
Calc 400
8.1 - 8.3 ph

I run a bare bottom sump , no fuge , BK 160 skimmer , two mp 40's , aqua illumination LEDs .

This is something that has me stumped as well as my closest friends . It is a 60 cube . The only thing I can think of is my live rock leaching phosphates but if they were then why is no algae growing?

This has been going on for about 9 months , my tank is 3.5 years old .

Your thoughts?

Reefers Digest

New member
I could only question the meters.

Maybe try a classic titration/colour change kit and see what you get.

Other than that... it has to be the rocks or food... but 1.8 just sounds too high to be right, with the other inhabitants still thriving and no sign of algae.


New member
Here is a Picture of the tank


I have tried both test devices. As soon I add the reagent to test sample , it turns blue. So I know for a fact that it is not the test kits and phosphates are present. For none of the inhabitants to be stressed is what got me stumped.

Randy Holmes-Farley

Reef Chemist
Premium Member
What are the units of that 1.8?

IIRC, the Hannas read in ppb, and 1.8 ppb is not high. :)

I don't believe that tank has 1.8 ppm phosphate.


Tank Tinkerer
Is it the Hanna 713 or the 736? I have the Hanna 736, which reads in ppb and reads phosphorus. I take the reading, divide by 1000 and multiply by 3.066 to get a phosphate reading in ppm. When I get a 30 reading on the test meter, that equates to 0.091 ppm phosphate. An 18 reading on the 736 meter would equate to 0.055 ppm in phosphate.


New member
I have the 713 and the units are in ppm . It is 1.8 ppm .

When I add the reagent to my test water , the test water turns blue indicating there is phosphates.. If my phosphates were low the tube should not change color correct? Something in my tank has to be leaching phosphates but is not detrimental to my system. Now I am really stumped!


New member
I had very high phosphates and little to no nitrates for the last few months. my sps would not grow. I bought some more at a frag swap and the new ones went from great PE and color to no PE and a couple bleached in under a week. Took a couple weeks to figure it out as I wasn't testing phosphate assuming it to be low. When I finally ran a full test I was astounded to see it around 2 ppm and 0 nitrates. Quickly went through lots of gfo and still working at getting it down but the sps has polyps out again and regaining color.

The key is, the zoas/mushrooms/xinia/frogspawn looked fine. gsp and all sps had issues.

Edit: I had no indications of algae issues. this was the reason i was not testing for ph/nit


New member
My guess would be the flakes. I read that some types of flake food can raise phosphates by as much as 0.4 ppm in one feeding. I wish I could find that article.... you may just not be seeing the ill effects yet.....


New member
I stopped feeding mysis for 4 days , but continued the omega flakes and pellets twice daily . My phosphates went down to 1.53 ppm .

Today I added GFO in a mesh bag under my return in sump . I am still leaning that the live rock is the culprit.

The one question I have is would the Texas holey rock centered in the pic covered in purple bonsai be leaching phosphate ? I know it was rock not native to reefs .

I tested the salt mixing bucket water And it read 0.001. The RO source water is at 0.000.


Your rocks may be holding a load of bad stuff after 3 + years. Uneaten flake food may be a big piece of your puzzle. Do you flush the rocks with a turkey baster?

Randy Holmes-Farley

Reef Chemist
Premium Member
FWIW, I don't think it matters all that much whether food is eaten by a fish or not or not. Something eats it, and that something releases most of the phosphate that was in it. There is generally much more phosphate in foods than creatures need (even people), so they excrete it back out again. People don't hear much about this, but just like fish excreting ammonia, they also excrete a lot of phosphate. :)

I agree that once phosphate is substantially elevated, it can take a lot of effort to remove it from the rocks and sand where much has attached to any calcium carbonate surfaces.

Also, all foods necessarily contain a lot of phosphate, even fresh seafood. Some brands contain relatively more than others however. :)
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ReefKeeping Mag staff
Premium Member
The bacteria from organic carbon dosing may be keeping the algae at bay on surfaces and/or you have a particularly good herbivore crew.

1.8ppm is very high. It's a problem for stony corals and will definitely inhibit calcium carbonate precipitation . I'd get a second PO4 reading with a test kit or another instrument and/ or test some mixed salt water to see what reading you get.

Perhaps it is offset to some extent by high alkalinity? Even if your alk is high, I still don't think it could offset 1.8 ppm PO4,though.


New member
Since adding the GFO my phosphates have climbed down the first week to 1.12 and just tested again this week and it is now .83 .

I have tested with a 4th phosphate colorimeter and all the tests are within .002 of each other .

I know science tells me that high phosphates are detrimental to any reef , but this has my local club stumped as well . Two fellow reefers both looked at my tank in person and both of them said this tank can't have high phosphates until they look at the digital read out . I have stopped feeding frozen foods all together and only feed twice a day , small amounts of Thera A pellets.

Any one have this issue before?

Randy Holmes-Farley

Reef Chemist
Premium Member
Which issue specifically? Phosphate above 1 ppm and thriving hard corals and zero algae? I've not seen anyone post that before.

I can readily believe the algae issue if the tank is limiting it some other way, but I'm surprised the hard corals do well. In any case, I expect they'll do much better at least in terms of growth rate at phosphate levels below 0.1 ppm.


New member
Futher proof what would devaste one reffers tank works in anothers!

Odd, but seems like the tests are accurate and the steps you are doing are making the new test seem legit!.

But still makes no sense as even 3-4 weeks 10% WC with tap water, horrible skimming, over feeding & overstocked should not even get you that high of PO4.

and you really are doing everything basically correct aside from maybe a TAD overfeeding, but not like its a lot