If you think 0.005 difference in salinity is the cause for your corals being that upset, I have a bridge in Brooklyn I'd like to sell you!
And I agree with Ben. Comparing your meter to one other just says one of you is wrong (unless the LFS calibrated their meter right there in front of you).
And to the OP and all the others that have trashed the hydrometer, I'll give you a different take on the situation.
I use a hydrometer all the time. But one of the first things I did was take my refractometer and calibrated it with proper calibration fluid. Then I tested my aquarium water with both the refractometer and the hydrometer. The refractometer said 1.025 and the hydrometer said 1.021. So I made a label that says "add .004" and put it on the side of the hydrometer.
Over the past 5 years I've used the hydrometer almost exclusively. But every 3 months at first, and less and less frequently as time went by, I would pull out the refractometer, calibrate it and compare it to the hydrometer. Every single time I do the comparison the hydrometer reads 0.004 too low.
So refractometers, if calibrated can read exactly right, but how often do you need to calibrate it? I found mine needed it every 6 to 12 months when I used it regularly. But I only 'calibrated' my hydrometer once 5 years ago and it has been absolutely the same ever since.
So I'll keep my cheap hydrometer and do the simple math of adding 0.004 to whatever the meter says. Oh, and how much less expensive is the hydrometer compared to a refractometer?
