I was reading through some guides about cycling a tank, and something this guide said surprised me: http://www.firsttankguide.net/nogoldfish.php
On a note against cycling with goldfish, he says, "Goldfish have different care requirements than do tropical fish, and if you keep the goldfish healthy, and then try to adjust the tank for your tropical fish, you will stress the filter and will cause the tank to at least partially cycle again - you could even cause the tank to completely cycle again."
I've never heard that you can make an already cycled tank cycle again... Or is there something I don't know about (I've only been in the planted tanks hobby for a few months). What instances would cause a tank to go cycling again?
Thanks in advance!