My 2 cents...
I would scratch the idea of cleaning any parts of your filters in tapwater for the time being. You need all the bio you can get from the sound of things.
Bderick67 is right about getting your 75/aro stable. No need to worry about messing with the 180 becuase your 75 is actually cycling now. Once the tank has cycled and the bacteria have established themselves on the media, then you can do just like Bderick67 said and swap tanks.
You would drain all water from the 75 into 180. Move the aro over to the 180 with its old water. Continue on filling the tank with fresh treated water, and it will be no worse for your aro than a regular large water change. Move your filters and equipment over and your done. (always a good idea to test new equipment before firing them up in a new tank with fish, ex. heaters filters so on)
If you have an established filter... you have an established tank.