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Discussion Starter · #1 ·
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 Expiry in 07/2022 so not out of date.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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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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Discussion Starter · #5 ·
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. :proud:

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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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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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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Discussion Starter · #8 ·
Not sure about your last question but also be aware that phosphate doesn't stick around in the water column like nitrates if you have an active substrate like aquasoil.
Ah, so I wonder whether the Amazonia soil is having an effect here? Does aquasoil tend to remove remove phosphate from the water or do something that affects phosphate test readings?

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
I slightly increased the phosphate dosing across my tanks and have found that I now get a mix of the yellow 0ppm (as per saltwater chart) and very light green 0ppm (as per freshwater chart) between the tanks.

My thinking is that a yellow 0ppm result means that there is absolutely no phosphate whatsoever in the water, and that the light green 0ppm reading means that there is a tiny tiny bit of phosphate detected that is nearer to 0ppm than 0.25ppm. Realistically it's pretty hard to tell between the 0, 0.25 and 0.5ppm shades of green on the freshwater chart anyway...

I got a response from API tech support which didn't help in the slightest:
"Thank you for contacting API Technical Services. We would recommend to match the color as closely as possible to the color chart. Every water chemistry is different so the depths of color can vary. Oppm would be appropriate for the phosphate result."
Take note of Seachem's excellent tech support please API!
 

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I find the phosphate test to be very frustrating. Even when you do have phosphates it is near impossible to see a difference between the first 2-3 green values.

But that is interesting, why do they have a 0ppm that is green but it can also be yellow?

I think a more accurate card for freshwater would be:
Yellow: PO4 = 0ppm
Light green: 0ppm < PO4 < .25ppm
Light green shade imperceptibly darker than above: .25ppm <= PO4 < .5ppm
Light green shade imperceptibly darker than above: .5ppm <= PO4 < 1ppm

so the freshwater card is WRONG for first light green shade. It is never 0, as in the case of 0 it would be yellow in color.
 

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Reading Phosphate test is easy but of course not super aquarte. Yellow you have no PO4, Greenish you have around 1ppm, the test will turn Blue above 2ppm but you shouldn't really go that far. The test is to give you a basic idea of what's going on with the water chemistry but will never give you super aquarte reading.
 

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Ok, thanks @ahem and @vanostav61 <a href="http://www.plantedtank.net/forums/images/smilies/proud.gif" border="0" alt="" title="Proud" >:)</a>

That is the conclusion I had come to (use the saltwater card with yellow = 0ppm, rather than the freshwater one) so appreciate the sanity check.
Bro, you need to use the Salt Water card for salt water aquarium and fresh water card for fresh water aquarium. Don't get stuck up on the initial color of the solution.
 

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Discussion Starter · #13 ·
Bro, you need to use the Salt Water card for salt water aquarium and fresh water card for fresh water aquarium. Don't get stuck up on the initial color of the solution.
So back to my original question then: How do you interpret a yellow freshwater test sample against a freshwater test card that only has shades of green and blue?

Other than the big difference for the 0ppm colours, the others are as close as you could hope to differentiate between the fresh and salt water cards anyway!
 

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So back to my original question then: How do you interpret a yellow freshwater test sample against a freshwater test card that only has shades of green and blue?

Other than the big difference for the 0ppm colours, the others are as close as you could hope to differentiate between the fresh and salt water cards anyway!
0ppm
 

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Discussion Starter · #16 ·
Exactly! :wink2:

And since the only real difference between the salt and freshwater test cards is that the fresh has very light green 0ppm and the salt has yellow 0ppm, this means we're in effect using the salt water test card to read fresh water results.

API support weren't much help (don't think I made it past first line customer support to anyone with any real product or tech knowledge though). But I would really like to know why there are different salt and fresh test cards, how salt water affects the phosphate reading, and why this is only really significant at the 0ppm end.
 

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Exactly! :wink2:
But I would really like to know why there are different salt and fresh test cards, how salt water affects the phosphate reading, and why this is only really significant at the 0ppm end.
Maybe it's more of a spectrum of the TDS. So the salt card is essentially for very "hard" water while the fresh is for "softer" water. So maybe if you have harder fresh water, it reacts more like the salt water card colors. Just reaching...
 

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Discussion Starter · #18 ·
Maybe it's more of a spectrum of the TDS. So the salt card is essentially for very "hard" water while the fresh is for "softer" water. So maybe if you have harder fresh water, it reacts more like the salt water card colors. Just reaching...
Good thought, like the logic! Only my water is pretty soft (6-7 GH and 0 KH). Unless it's a quirk of soft water or having 0 KH somehow?
 
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