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My best investment , is the time I spent to get my wife to be sort of involved with my hobby!!
Refractometer Calibration fluid - $3
With it I determined that 3 hydrometers were wrong and my refractometer was almost .003 off! This is a huge number, especially if you are one of those people who run it +-.002. That means you could be up to .005 off! Imagine thining you are at 1.022 ("I like to run it light because it is good for the fish"), and in reality you are at 1.019. That might explain all of those coral problems you are having.
nice, they don't use the adjustable pinwheel anymore to compensate for the glass thickness?