[slief]

Originally Posted by homer1475

I also use the phosphate(Not ULR), and Alkalinity.

Question on the ULR meter. If the regular phosphate meter goes down as far as .01 why the need for the ULR if optimal levels of PO4 .1?

Because .01 on the standard one means you have greater than .05 ppm and probably closer to .10
I’ve done some side by side testing and the HI713 was reading 0 ppm while the HI736 ULR was reading .07ppm

The range we test in is ultra low range and the phosphate tester is a low range tester meaning the lower the Po4 levels, the less accurate is becomes. It has an accuracy of +- .04 ppm at low ranges which means at low range, it could be off by several PPM. Notice the “marine” designation on the HI736 ULR and the lack of it on the HI713 phosphate tester? The HI736 is the one that is ultimately designed for our use and what Hanna recommends for marine aquariums and why it has that nomenclature on the tester. At ultra low ranges, it can be off even more. If you want accurate phosphate test results, the ULR is the one you want.