You can add it to the tank before adding water (or are you talking about transferring the sand from your other tank?). If you’re transferring it, I would give it a good rinse in some tap water and then rinse with either RODI or saltwater, you don’t want to transfer all the detritus to your new tank. Either way, would still be ideal to add to the tank before adding water, if that’s not possible, use a little cup and add it one cup at a time to avoid clouding up the water (will still cloud up a little, but much less than if you dumped it all on at once from the top of the tank).