i wouldn't add the sand to the mixing tub, as it prolly would make the sand a bit salty lol.
1. i would just rinse the sand the best you can. to rinse it you put it in a bucket and fill it up and let the water overflow the bucket until the water overflowing is clear.
2. hook the fuge up
3. have a beer
4. add the mixed salt water to the system, should fill the fuge no problem. could add it to the fuge. shouldnt matter where you add it. it should fill up. if it doenst you have a problem
5. turn off the powerheads to the fuge
6. add the sand and let the dust settle
7. turn on the powerheads
8. have a beer
9. enjoy the fuge :0
10 REPEAT STEPS 3 AND 8 AS NEEDED !!! just remember patience is a virtue.
good luck and enjoy