EDIT: So, I am a moron. I have the Hanna HI713 Low Range Phosphate Checker, not the HI736 Phosphorus ULR. Bought it in an order with a bunch of other equipment and actually bought the wrong one. In the end it was fine for two reasons, (1) I had to test my rocks curing in fresh RO/DI and (2) my levels were high enough to need a tester with a range higher than the ULR. Eventually, I will get the ULR as well and probably look to get one used.
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