Not to beat a dead horse, but I run into a lot of people who don't understand the cycling process for aquariums so I am always going to double check.

It has nothing to do with keeping the tank clean or having a filter - for the water to stay healthy all the time there has to be a large colony of good bacteria that eats the waste of the animals in the tank as soon as they poop it out. If there's no good bacteria or if there's not enough of the bacteria, then the ammonia and nitrate rockets up which will kill fish and amphibians very quickly, sometimes within an hour. Knowing where your tank is at in the cycle is vital, and knowing what the ammonia, nitrate, and nitrate levels are is also extremely important. Petsmart and Petco will run water tests for you for free if you can't buy a testing kit, just bring in a little container and take it to the fish counter. And to learn about how to cycle a tank, there are some great guides out there, like this article from the Cichlid Forum:
http://www.cichlid-forum.com/articles/cycling.php