an empty tank will somehow cycle out. i guess its algea and other natural bacteria that naturally grow??
with my first tank, i let it sit with the filter goin for a month and with no additives it completely cycled on its own. i didnt know much then, but it was ready for fish ^ ^. the other 2 tanks were filled with water from the already established tank. filter gear and decor were also added. those 2 tankes were ready in 2 weeks at the most. ive never used fish to cycle. it has something to do with reverse osmosis, im not sure what it is, but it works
very good post btw. im sure many people will find this very helpful!!