Upgrading to a 75 gallon from a 45?

The tank will most likely already be set up without water in it so can someone tell me where i get the water to fill the rest of the tank after i empty my 45, does that already made water have the bacteria needed to support life. what ratio should i use between made sw and store bought pre made. And if someones willing to tell me step by step on how to do it, and i got a problem is that i want to the new tank to go where the 45 gallon is because that outlet is set to a generator. Any ideas? Any experience? i have corals,fish, one crabs snails and some other stuff and my least favorite thing to move... a long spine sea urchin.
thanks

 

Oh yeah, what do i do about sand, there's not enough in my 45 to cover the 75 gallon tank and i don't wanna crash my system.

 

A) You are going to want to use new sand. Too many problems can occur with using old sand, as too much detritus can accumulate in the sand. Stirring that up can cause an ammonia spike, crashing your new tank.

B) The water itself contains almost no bacteria. The live rock is going to be your source of already established bacteria. So don't worry about using old or new water as it won't really matter much. Just make sure to keep the live rock submerged in water (don't let it dry out), and match the salinity and temperature of the new water with the old water.