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When I bought my hydrometer at the LFS they told me to bring in some of my tank water next time and they'll use their refractometer on it and tell me how far I'm off. That way I can compensate. Maybe your LFS will do the same.
I recently purchased a refractometer and found my was reading hydrometer was .002 too low. 8O
Its a very good purchase, especially for a qt tank setting where you are doing a lot of water changes. Or maybe you can at least borrow one to find out your true mix.
well i just tested my refractometer tonight with some ro/di water it haqd been a long time since i did that and boy oh boy was i shocked it was reading 35pp before i recalabrated it and after it was reading 40 ppm a difference of 5ppm it was off. man that makes me mad. i have no idea how it got off that much. but now i will be testing it about once a month or more. i thought you could set it and never have to test it again.
Update: I purchased a third hydrometer. a floating, bobber looking kind. it reads the SG as being 1.020. so i will assume for now that if 2 of the three are reading the same. than those are the most accurate.
the LFS hydrometer (same as my deep six) read .003 lower than mine did in their water.
so who knows.
What is the danger zone for fish and inverts? how far off of 1.025 can I be and still be ok?