well shd be no prob... in fact thats what i just did when i transfer frm old to new tank. I ran new water for a day, put in my old liverocks next day n then zoos n mushroom the next day n so far no prob at all with them. In fact i put in 3 "old" fishes another 2 days later... so far nh3=0, n02=0 , n03 =5