Put some in a bucket and run water though it. Keep adding water and stirring until it is clear. It will take some time, but it is worth it. Once you have clear water add the sand to the tank and repeat.
get a short piece of PCV pipe and drill a bunch of holes in it...cap one end and put a hose connector...available in the same department of the home improvement place. Then stick the washing wand into the sand/gravel...connect a hose...and let it run for a while.
the bucket and a hose method is a tried and true one. I just set up my 75 with sand two days ago. it took one hour to rinse 100 pounds and today it's crystal clear. Sand takes a little more effort than store bought gravel but is definitely worth the extra work.
seriously? two days? What kind of sand did you use? I use PlaySand from Lowes (which most people say is the dirties sand) and 100lbs took an hour with a hose and a bucket.