Just bought a new API Cal kit and it reads 500. I was using real natural saltwater from the Gulf of Mexico so rechecked with my trusty salifert kit and measured 440 which is the norm. Gotta return the API kit for a refund. But used another batch of the API cal. test before and it was dead on with Saliferts and much easier and quicker for me to use as the color change is easier for me to pick up.
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