How can you recommend dry rock in one sentence and recommend live sand in the other? Mil26 already has live sand, and he will be seeding the new dry sand he buys just as he will seed the dry rock. The "live" sand you are telling him to buy is sand that had bacteria on it, which has probably already eaten up all the resources inside the bag, and now are dead bacteria anyway. You may have purchased live sand, but that doesn't mean that is what Mil should do. Live sand could in fact lead to a cycle for Mil26 if it is not rinsed throughly because of the die off in the bag.
As for the phosphates argument, I disagree. When the sand is rinsed until clean, the only remaining phosphates I feel that would leech from the sand would be what is naturally inside the sand. Chances are his water contains chlorines and would nuke anything natural anyway. This is sand we are talking about here, whether he bleaches his sand or not, he will see no negative effect from re use after throughly rinsing until clear.
Also, you need to keep your live rock submerged in water during this move Mil in order to prevent die off on the rocks.
I am sorry, perhaps I should have been more clear.
If he currently has Argonite based, crushed coral based live sand he should replace it. Yes, it will have significantly higher phosphates than new live sand.
The difference in phosphates in sand that has been in an aquarium for years, vs sand that came from the ocean is significant. All sand containing calcium carbonate Adsorb phosphate, overtime they become less effective at it, and eventually will even start leaching it into the tank.
I am not talking about phosphates on the outside of the rock from decaying organism I am talking about phosphates that have been picked up through adsorbtion into the calcium carbonate. No simply rinsing under water will not remove that. The danger it causes is something called Phosphate Leaching.
Also adding dry sand, will greatly reduce his biofilter for months compared to live sand.
So to be clear... I am not worried about Phosphates you can rinse away with water. I am talking about the Phosphates that through adsorption are inside the calcium carbonate.
Why did I not recommend replacing the Rock Also?
1. Cost
2. It is being used to prevent a cycle
3. Seeding his new dry rock.
4. If phosphates do become a problem he can always cook it at a later date.