I think javatech said it all,, hydrometers vary for some reason..recently , i compared my salt content with my lfs i was using the instant reef hydrometer and found my salt off by .05 which brought my salt from what i thought was 1.024 to be 1.018..he uses a refractometer ..i'm saving up for a refractometer currently i'm using red sea's hydrometer..which confirms my lfs levels..