New Fish Tank

Hi guys!! How's it going?

Say, I just bought a new fish tank for my kids as a present for Christmas.
It was used, but I couldn't resist it. 400 gallons!!! I paid $1,000 for it
and the oak stand. It is square in shape and about 6 feet long on all sides
and 2.5' tall on a 3 feet tall stand. Viewing is about eye level for an
adult.

Wondering how soon I can put fish in the tank. I have over 600 lbs of live
sand that was sold with the tank and we kept 30% of the old water and added
70% new RODI. The tank has a huge skimmer about the size of an old gas pump
that sits externally to the tank. The tank is turned over at least 40 times
per hour according to the guy I bought it from due to the humongous dual
pumps also running externally to the system. The sump for this thing has a
capacity of 250 gallons and is under the tank in the base of the stand. You
guys have to see pictures of this!!!! It's freaking awesome!!!

So!! How soon can I add fish and more liverock? The chemistry is awesome!!!!