So if I were you this is what I would do. Take out 55 gallons of water from your 150. Take all the water( if the gravel matches add it to) from the 55. Add that to you 150, with your fish( stir up the water in your 55, so that it looks dirty use that water). All the muck and gunk, contains nitrifying bacteria, which is the building block. That will keep ammonia, nitrite and nitrate in check. Make sure your temps are with in 3-6 degrees of each other. So basically it will be as if you did a major water change. If you have cycle, stress zyme or bacteria supplement. Add that for the next 4 days.