I have all three and the phosphate is by far the worst of the them. Results have huge variations when performed back to back; and even when performed side by side with another Hanna checker. Hard to trust the results. The ULR phosphorous checker is a better option even though you have to convert to phosphate and change the units of measure.
The Alk and Cal both work great for me. They are in the same ballpark as results from chemical kits like the RSM series. Cal is a more involved test but once you get it down it's not really a big deal.
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I have to go out and buy more snails for my hermit crabs.
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