I've always struggled with the Red Sea Phosphate kit, but I'm also having big discrepancies with Hanna readings on Nitrates
Red Sea phosphate has been telling me I'm around .12 - having been around .03-.08 for months and months before that. Okay fine.
Hanna says my phosphates are .28 across two tests.
Red Sea nitrates says I'm somewhere between 20 and 50 - let's say 35
Hanna says I'm at 14.1
Who do I trust?
Red sea seems all over the place on other kits too - their standard alkalinity and alkalinity pro kits read differently as well.
(for what it's worth I'm doing ~23% water changes daily until I can bring these levels down.)
All of these kits are very frustrating to deal with.
Overall I consider the Red Sea Phosphate test pretty good, but there are a couple significant limitation users need to be aware of.
First the good news. At low phosphate levels the RedSea test reagents produce significantly more color contrast than Hanna’s best ULR testers, and this results in a more sensitive, accurate, and consistent result. The Hanna Phosphate ULR stated accuracy is a disappointing +-0.020 ppm plus an additional +-5% of the reading that speaks for itself.
But the RedSea test has a serious Achilles heel. The test saturates around 0.120 ppm. This means that any concentration over 0.120 ppm, will only read approximately 0.120 ppm. I believe this is the situation you were dealing with. When the test results are over 0.100 ppm, the RedSea results have to be discarded!
RedSea has a procedure in their instructions to dilute the sample to increase the range, and they recommend DI water. But the DI water should be tested by itself to verify it’s free of phosphate. My city water comes into the house at 0.500 – 1.000 ppm phosphate, so the DI system must be working properly to result in phosphate free water product water. You can also try bottled distilled water if the DI is a problem. Personally, I do not like the dilution process, and if the phosphate reads over 0.100 ppm, I believe the Hanna ULR test is the way to go.
The problem with Hanna is at levels below 0.050 ppm or so, no matter how careful I am, the results are typically not very consistent and repeatable. The Hanna test also seems very sensitive to contamination inside the cuvette. Good cleaning and rinsing with test water several times is required for good results. This also applies to the RedSea test.
The second RedSea issue I’ve found is with the reagents. I once received a kit where the results were always zero. Something was wrong with the reagents. I purchased the kit online, and it’s possible the kit was tampered with. If I’m not sure the kit is working properly, I will spike the 17 ml test sample with “Hach 256949 Phosphate Standard Solution 1 mg/L as phosphate.” Adding 0.17 ml will raise the phosphate approximately 0.010 ppm, and 0.34 ml will raise the phosphate approximately 0.020 ppm. If the test picks this up properly, I know its working.
