You can fill up the 125 with all tap. Just make sure that you add dechlorinator and bring the temp up to where it should be. Once you transfer the fish over immediately transfer over any decorations and filtrations. You might see a small cycle, but you should be just fine.
I need help tryong to setup a new tank. I have 2 55gal tanks up and running and I want to use everything from those tanks in my 125gal. My biggest question is how do I re-use the water so I don't have to cycle the new tank again. Or do I still have to do that? Also, what would be the best way to transfer all the water from the 55gal tanks to the 125gal? I will placing the 125 in place of the two 55gal. Hence I will need to move the 55 to put the 125 before I fill it up. I don't have much storage containers to store the water while I maek the transfer. Also, how much tap water can I use instead of the cycled water, that way I don't have to store all the water, because that would be 110gal of water.