Since you just started (Yesterday at the time of this post), and your tank is small, I'd suggest getting the water out and refilling with RO/DI water, either from a LFS or from a water unit such as at a grocery store. The LR could be kept in a bucket (Probably with the water currently in the tank) heated and agitated while you get the CC out if you want and put in some sand.
Since your tank is small, you could get by for now by buying RO/DI water, but you'll probably want a larger tank in the (near) future, and having your own RO/DI unit is very highly desirable.
I don't think you've screwed the pooch irreparably, and you've caught this in time to more easily rectify it.