I use Hanna low range phosphate tester. Highest it goes is 2.5. I previously had fish die that was at the LFS for over a month. I QT all my fish in a cycled tank, it is kept running with live rock, to observe for any scratching or flashing. I only add meds if they show signs of disease. Havent had to add meds in over 2 years. And it sits for months just running without any fish. I do a large water change before adding new fish. Fish was eating like pigs for a couple of weeks, then they hide in the shadows and die after not eat for a day or two. This happen 3 times with fish that I watched at LFS that was there for weeks with no signs of disease. They were eating like pigs when I brought them. I always ask to see fish eat before purchasing them. After the last time it happen, I did a 100 percent water change and phosphates was still over 2.5. So I'm thinking very high phosphates was leaching out of the rock and killing them. The fish I treated did acted the same way. Basing my previous post on my experiences.