IMHO, using RO water to rinse sand or rocks is a total waste of good water. If you are that concerned, give them a final quick rinse in ro/di and be done with it. I never have. It is ready to use as soon as you have rinsed it well.
IME, you need to rinse it very well to avoid clouding the tank for days afterwards. Tap water is just fine for this.
Here's how I do it:
I use the utility sink in my fish room. I made a standpipe for it that sticks up about 6" from the drain. Put your sand, a few pounds at a time, in a clean 5g bucket. Place bucket in the sink and start the water running. Stir the sand continuously - I use a 4" plastic putty knife. Allow the water to overflow the bucket into the sink. Don't worry about any sand that might go over the top too - the standpipe allows it to settle out while at the same time allowing the fines (dust) to go down the drain. When the water in the bucket runs clear, you are done with the first batch. Pour as much water out of the bucket as you can and then remove the sand to another bucket (the putty knife, again, works well for this). Repeat the process until all your sand has been cleaned. You will have some sand (a lot?) in the sink. Carefully remove the standpipe and allow the water to drain while trapping the sand in a corner and then transfer that sand to the final bucket as well.
Aren't you glad all that sand didn't go down the drain? Not only is it a waste of money, so is the call to the plumber for the clogged drain! (don't ask how I know this, just take my word for it)
If you are really concerned, a final rinse in RODI can be done - I've never seen the point, myself.
hth
