I am going thru a similar situation right now, I had a 25 gallon tank and moved all the inhabitants into a 75 gallon tank I put the old filter and a new Hob on the new tank.
I thought every thing was going well I had used new substrate in the tank and had 3 times the tank volume per hour circulating. My water was crystal clear and I had only a couple of little fish in the tank, after a couple of weeks I decided the tank was ready for more inhabitants and put in 4 more fish, a week later I added 15 small schooling fish, with in a few days my water turned milky cloudy and my amonia has spiked frequent water changes are doing nothing. After researching I am convinced I have way over loaded my bio too quick and am experincing another cycle, I have moved some of the fish back to the 25 gallon and am reducing the feeding .