Okay, it appears that I misunderstood? Is the water that is in your rubbermaid tub tied in to your system in any way? Is it part of your sump or fuge or top off or anything? If it's just a big bucket of water that does not connect to your reef, then it could have waited the 18 hours. Sorry to be so alarmist, it sounded to me initially like you had already put it in the tank.

If that is the case, that the water is not connected and just sitting idly by, then you should not have any problems with your reef chemisty once the proper salinity has been achieved (other than whatever problems you were might have had anyhow even if you had mixed the salt correctly). Hope that is clear.