If you have a decent biological base (cycle) in place you should be able to add live rock and with the exception of ant crap on the rock dieing off, there shouldn't be much of a spike. Your existing bio flora will absorb a lot of it....I am going to be increasing from my 29 to a 75 gal and already have the majority of my rock either in my tank that is running, or circulating in a bucket, out of an established tank, and I don't anticipate much, if any cycle to be involved....