Couple questions

I have a few questions regarding my tanks.



I tore down my old hospital tank to take to work. I removed all the water and completely scrapped everything down to look brand new. It sat without water for 12-18 hours. I then moved it to my office last night and made up salt water and i set the entire tank up this morning at work.



I am wondering how the cycle works. Do i have to cycle the tank again cause the cycle was stored in my rock work or does the tank hold my cycle cause it was up and running for a couple months before.





Last question is i converted my freshwater 30 gallon to my new QT or New arrival tank.



I dont want to put rocks or sand in it just Bare bottom tank and only put water in it for when i order fish online or purchase a new one here in town.



So how does a cycle work on a tank like that? Do i have to cycle it everytime i put water in it again? I plan to only run 15 gallons in it and an air stone nothing else. Thus if i need to run copper for sick fish i can but that will be last resort.



Do i cycle this tank every time i add water or once it cycles once and i empty it am i ok to add water and add fish that night??