I use a Tropic Marin phosphate test kit and it works fine. Basically it tells me if I have phosphates. It is pretty accurate (I think). I dont really care if my phosphate is at .02 or .002. It's a less expensive kit.
I have heard online at RC that many test kits are notoriously inaccurate especially for measuring phosphate levels. Any brand recommendations please for a good test kit?
Instead of inaccurate test kits, I think that many people do not understand the principles involved, and then think kits are inaccurate when they may be perfectly fine. That does not mean that all are accurate, and some certainly may be, but I see no reason to think them worse than other complicated types of kits, such as nitrate.
The issue seems to be that folks know that algae needs phosphate, and when they have a huge algae problem they incorrectly assume there must be high levels of phosphate driving it. Yes, there must be a lot of phosphate available, but if algae sucks it up as fast as it is delivered from foods, the apparent concentration may always stay low or undetectable. That is exactly how macroalgae is supposed to work to reduce nutrients, and it also happens that is how problem microalgae works too.
The goal is to intercept the phosphate before the algae gets it, regardless of what levels you might detect with a kit, perfectly accurate or otherwise.
Also, some test kits do not read especially low in concentration. That does not make them inaccurate, it only makes then useless if monitoring very low levels of phosphate is your goal.
FWIW, in my testing the Hach phosphate kit (PO-19) is quite accurate down to about 0.03 ppm, below which it begins to be hard to accurately detect the color variation.