API phosphate test kit - wrong colour??? - The Planted Tank Forum
 
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post #1 of 7 (permalink) Old 09-01-2020, 02:03 PM Thread Starter
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API phosphate test kit - wrong colour???

Dear All,

Your help regarding reading the results from an API liquid phosphate test kit please! A picture says a thousand words so:



As you can see, my test tube is a light yellow colour, but the freshwater colour chart goes from light green to dark green. I take it that this probably means 0 ppm phosphate, but the chart shows that even a zero reading should give a light green colour, not yellow. However, on the saltwater colour chart (the water sample is from a freshwater tank, not marine!), the zero reading is actually supposed to be yellow:



So what on earth is going on here? Any ideas please? I'm pretty experienced by now with using these test kits and have followed the instructions to the letter:



I have also tried adding a drop of Seachem Phosphate into the test water and confirmed that the test tube does go a dark green colour, so I don't think it is a faulty test kit. Expiry in 07/2022 so not out of date. I know some people report faulty test kits, but I'm also aware that user error is far more likely. But I can't see what I could be doing wrong? I've done 20+ tests now on 4 different tanks and pretty much always get the yellow result. I did overdose* a tank I was cycling on one occasion and that's the only time I got a hint of green.

*compared to my usual dosing which is around Seachem's recommended 'beginner' dose

Why is my reading yellow and not light green as per the freshwater colour chart?

Is it just because the API test kit has such low resolution at the low end? Does anyone else get similar results? I am doing something stupid?

Ideas please! Thanks, James


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post #2 of 7 (permalink) Old 09-02-2020, 05:36 AM Thread Starter
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Anyone?


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post #3 of 7 (permalink) Old 09-07-2020, 12:50 PM Thread Starter
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Bumpity-bump!

Does no-one out there have any experience with API phosphate test kits???


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post #4 of 7 (permalink) Old 09-07-2020, 01:48 PM
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It is the same color as the reagents you put into it, so you are right. You have 0 PO4.
I did this same test a couple days ago with the same result.
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post #5 of 7 (permalink) Old 09-07-2020, 02:35 PM Thread Starter
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Thanks Ben, makes sense. No colour change from 'raw' reagent colour so they've not reacted with anything, hence must be zero phosphates in sample.

But then why is the 0ppm colour on the freshwater results chart slightly green, and not yellow like the saltwater one, I wonder?


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post #6 of 7 (permalink) Old 09-08-2020, 06:45 AM
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Not sure about your last question but also be aware that phosphate doesn't stick around in the water colimn like nitrates if you have an active substrate like aquasoil.
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post #7 of 7 (permalink) Old 09-08-2020, 01:28 PM
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I recently bought a phosphate test kit and tried it on my tank and tap water just out of curiosity (I use remineralized RO, so the tap doesn't matter). I noticed my tank water read 0ppm while remaining yellow, but the tap read 0ppm while turning that light green. I think it might just be residual amounts that are reacting, but for all practical purposes are 0. If you have some dry potassium phosphate fertilizer you can create a known ppm solution of phosphate to check that your test kit is actually working and accurate.

-AM
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