When I set up the new tank for the dwarf puffers, I really wanted an instant cycle because I was buying a new tank one day, and adding fish the next. Knowing they are very sensitive to ammonia, nitrites and even nitrates, I took every precaution.
I cut a piece of the sponge from an established aquaclear 70 to fit the aquaclear 20 I run on the 10 gallon. I put a sponge on the water intake so the puffers wouldn't get sucked up.
I put the rest of the sponge in the tank too. I also used a nylon full of substrate from an established tank. I put it in the nylon because it was a different color and I wanted to be able to remove it later.
I added plants, of course. I have added more since.
I also used water from an established tank (though apparently it doesn't help, I did it anyway) and added a bottle of colony bacteria.
I never had any ammonia or nitrites in the new tank.