I recently purchased a new bag of KNO3 from a supplier and over the course of 3 weeks my tank has turned green!!!!!! Nothing else has has changed as far as I am aware except the KNO3. I have purchased from this supplier before without any issues. So tested a teaspoon of the powder in 1/2 cup of water with my phosphate kit and got a reading close to 5 ppm. My test kit is pretty old (around 6-8 years) so possibly is a false positive, but I wouldn't have thought there was any PO4 in KNO3.... The supplier is positive the powder supplied is KNO3 so I am at a loss as to what to do now... keep dosing with it or get some more from somewhere else

Cheers - Peet

There should be 0 phosphate in potassium nitrate.

If you are reading correct or near then I would get rid of the lot as clearly there is a mix up and you have no idea what else may be in the mix.

Algae problems are often caused by an excess of phosphate over nitrate so I wouldn't have thought that a solution of your nitrate at a teaspoon to a glass of water which is only giving 5ppm in amongst all that nitrate. Plants need phosphate and that is a very small amount. I would doubt that is the cause of the algae. Many cheap fish foods will leave a residual of phosphate.

Hi Alan thanks for your response, I'm not sure I'm understanding you though. At the moment I don't think I'm adding any no3 as I think its all po4... Hence there is an excess of po4 over no3. I have run out of nitrate test unfortunately so can't verify

You could try a flame test. KNO3 will burn violet-red whereas PO4 will flame blue-green

Ok, flame test no good.... possibly because both have potassium in them.

But I took equal quantities of my Kh2PO4 fert and new bag of "KNO3" in equal volume of water and tested both with the PO4 test kit. Here are my results:

"KNO3"

Turned royal blue indicating > 5mg/L PO4 content

pH = 6.4

KH2PO4

No real colour change indicating 0 to 0.25 mg/L PO4

pH = 4.6. This is correct as H ions create acidity

See image above for tests... "KNO3" on the left, KH2PO4 on right. So now I am stumped. The pH of the "KNO3" is correct but the test result is not. I guess for the minimal cost of the fertiliser I will just replace it. Only issue is the supplier wont post. Anyone else have a 1kg of KNO3 they want to sell???

You said you had purchased a new lot of KNO3 so I assumed you were using it as a source of nitrate. My comment was to point out that if you did use it the amount of phosphate in it was minute and since plants need some phosphate as well as nitrate it would hardly make a difference.