It is a good idea to check a hydrometer at least every 6 months against a refractometer. In my experience most hydrometers are somewhat accuracy (+/- 0.003) for the first year or so, but wander off as time goes on. I have two hydrometers and they are off by 0.006 and 0.008 which is plenty enough to cause some serious issues in the tank! Both of my hydrometers read lower than they are, so the one reads 1.026 when the water is actually 1.034!
I like to use hydrometers for daily use though because it is quick and easy. I just use a Sharpie to mark the numbers I like to use. Rubbing alcohol will take the Sharpie off to re-mark when I re-calibrate.
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Originally Posted by nanomano
I am wondering if the effect is the same with soft and hard corals
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Corals don't tolerate hyposalinity, so there should never be a great swing. Some LFS keep their salinity at 1.020 or 1.023 though and I have never had trouble swinging from that to 1.026 or going the other way.