I only use seachem purigen and Chemipure in my filter. No activated carbon.

Thank you for your help posting that link.

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Here's a way to make 1.0, 2.0, 3.0, 4.0 and 5.0 ppm PO4 reference solutions:

Add 1.433 g of KH2PO4 to 1 L DI/RO water. This makes a 1000 ppm PO4 solution. (It's really a 1000.09 ppm solution.)

Add 1 mL of the 1000 ppm solution to 9 mL of DI/RO water. This makes 10 mL of a 100 ppm PO4 solution.

Add 2 mL of the 100 ppm solution to 18 mL of DI/RO water. This makes 20 mL of a 10 ppm PO4 solution.

To make a 1.0 ppm PO4 solution:
Add 1 mL of the 10 ppm solution to 9 mL of DI/RO water. This makes 10 mL of a 1.0 ppm PO4 solution.

To make a 2.0 ppm PO4 solution:
Add 2 mL of the 10 ppm solution to 8 mL of DI/RO water. This makes 10 mL of a 2.0 ppm PO4 solution.

To make a 3.0 ppm PO4 solution:
Add 3 mL of the 10 ppm solution to 7 mL of DI/RO water. This makes 10 mL of a 3.0 ppm PO4 solution.

To make a 4.0 ppm PO4 solution:
Add 4 mL of the 10 ppm solution to 6 mL of DI/RO water. This makes 10 mL of a 4.0 ppm PO4 solution.

To make a 5.0 ppm PO4 solution:
Add 5 mL of the 10 ppm solution to 5 mL of DI/RO water. This makes 10 mL of a 5.0 ppm PO4 solution.
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Pardon me for my ignorance...can you please tell me which solution to make per the lists above?

Make the one that will be easily detectable by your test kit.

I have a test kit of 0.0-5.0mg/l, therefore I would go to make 10ml of a 100 ppm of PO4 solution?

Its just showing you the dillutions.

You start out by making a master mix of 1000ppm.

From there you are dilluting down to 100, 10, 5, 4, 3, 2, 1 ppm solutions from the 1000ppm 1l solution with RO/DI water.

If you really are intent on testing PO4 you can use the 1000ppm solution to make stock reference solutions. Get some empty bottles and store stock solutions (5,4, 3, 2,1 ppm ). From that chart even making 1/2 solutions (4.5, 3.5, 2.5, 1.5, .5) to be even more accurate. So before you test your tank water have plenty of vials ready. Then test your tank water and stock solutions at the same time. Then you can match to the closest solution. The point being that you are not using a color chart but a closest match to known solutions.

So to be partially acurate at 1/2 ppm intervals from .5 - 5 you will be making 10 seperate solutions plus the tank water so a total one time test of 11 vials.

I can't see being accurate by making a solution of say 3ppm. Checking the color chart and going off of that. Most if not all of the hobbyist grade PO4 tests are
1) innaccurate
2) have poor color charts to determine PO4 levels
4) Have wide gaps in the chart once you are over 1ppm.
API - 0 - 0.25 - 0.5 - 1.0 - 2.0 - 5.0 - 10.0
Salifert - 0 - .03 - 0.1 - 0.25 - 0.5 - 1.0 - 3.0
5) Matching colors to these charts is problematic at best.

This is why I can only see making as many reference solutions as possible and testing all at the same time. Otherwise IMO you are going to be wasting your time.

Your readings can vary for alot of reasons. Some tests require shaking the bottle vigorously prior to use. What happens if you forget? What does that do to the solution the next time you use it? So if you only do a one time calibration and continually go off of that reading, IMO you are just fooling yourself. API's Nitrate test kit comes to mind. I found Silifert's PO4 to be the easiest but still very difficult to get consistant readings from known stock solutions due to the damn little spoon and large granual reagent.

What I"m trying to get at, and I'm not trying to dissuade you from testing if you truly feel the need to. Hobbyist test kits are very problematic and to test properly with them requires a great deal of effort to calibrate them to get an accurate reading. Hope this helps and good luck.

Unless you enjoy doing testing, calibrating the test kits, calculating how to correct what the test kits tell you, etc. you are much better off using the test kit for a book end, and not testing. If you have a problem the plants will very likely tell you about it much more accurately than the test kit will. As far as I can see you don't have a problem, so relax, enjoy pruning the plants, watching the fish, and keeping those books upright with the test kit.

+1

Very well stated.

when i first set up at tank i test frequently for the first 3 months or so

after that i only test when there are issues i can't get to the bottom of, e.g. fish death for unexplained reasons

i'm actually more likely to test tap water than tank as well, i can tell you that my tap water is 7.2pH, kH and gH < 20ppm, 0 nitrite, 0 nitrate but couldn't tell you what any of my tanks are sitting at right now although i know what i'm aiming for in my malawi tank

test kits are essential for setup and issues, otherwise why bother?

Are you using ada soil? If you are, it releases P into the water.

I use lamotte test kit, had same problem, i dont know why but i cut down on my po4 to once a week about 2 month ago, so far so good, i did not caliprate the kit, i have the water company test results saying 0 po4, which my kit say. am not an expert, i dont pother with callipratin, i just use the ket to tell me if things are very low, ok, or very high, you can calliprate if you need acurrate results, i did caliprate in the past but really did not make much of a diffrent.
if your kit above the sclae you are most likely at the high end.