I suspect I have seen the buildup of organic material affecting test kit in my tank as Tom described. I have a VERY heavily stocked planted discus tank(100gallon), into which I dump more than 3 ounces of frozen blood worms a day. It is the NO3 test(AP kit) in my case. I was changing 50% water weekly for a while. Normally the NO3 test sample should be a clear yellow after the first reagent is added. But I was getting an orange color, it was like I already got significant NO3 reading BEFORE the second reagent was even added! Actually the color became lighter with the second reagent added, and what is interesting is, the final reading took much longer to reach than usual. I took another look at the sample the next day, the reading was much higher than it was 30 minutes after the test. BTW, my AP phosphate kit is toally junk, reagent probably expired from the beginning. It gives me a totally different color than the chart range!
I have increased W/C to 3x60% weekly to keep DOC and nitrate in check. That's what I pay for wanting so many fish. So my tank is reset very frequently(3/week) with Tom's method. The plants are doing very well and there is little algae. It is a little surprising considering I have ample supply of NH4 from fish which should favor algae, maybe my wet/dry filter helps, Tom?
I have increased W/C to 3x60% weekly to keep DOC and nitrate in check. That's what I pay for wanting so many fish. So my tank is reset very frequently(3/week) with Tom's method. The plants are doing very well and there is little algae. It is a little surprising considering I have ample supply of NH4 from fish which should favor algae, maybe my wet/dry filter helps, Tom?