I'm betting there is some residue on your hands(or gloves) that is inadvertently being transferred to the cuvet before testing. Do you use a clean/sterile cuvet each test?
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Rico's Reef - Born 07/15/2006 - 75 gallon reef, 29 gallon sump/fuge
Polly's Ocean - Born 11/24/2007 - 10 gallon mini reef
Rico's Moneypit - Born???(coming soon) - 240 gallon mixed reef
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