Lots of variables there. Depends on fish/bio load and size of old/new tanks and amounts of liverock etc. I transferred my 72g tank into my 200g with no problems or spikes whatsoever and I had a huge bio load, but a ton of liverock too. Sump was also kept from old tank to new tank. Takes some planning and proper procedures to move things along in a timely and effective manner. As far as keeping GHA and bryopsis out indefinitely, good luck with that. Each time you add a frag your going to get something else with it. No matter how careful you are it will always find a way in. Better to have a plan to deal with it effectively IMO. I wish I took pics of my frag tank covered in bryopsis so high you couldn't see the corals. I pulled out what I could, but 20 emerald crabs in a 40g tank, completely devoured any trace of brypsis in under a week. What inspired me to do that was a previous explosion of bubble algae in my 200g. (I attributed to re-homing 6 tangs all at once that must have been keeping it in check before) I added 20 emeralds to that. they went to work but not fast enough. Added 20 more. Same results. 20 more and that was magic (So a total of 60 crabs/200g). Total of about 5 or 6 weeks from the first purchase of 20, and no more bubble algae. I then relocated many to other tanks but I still have at least 20 in there to keep a good maintenance going.