Close BeerNinja; go check the article link I posted in your thread
But to clarify Enki's explanation (and so I get to use toilet word again *grin*), fish give off nitrogenous waste in their pee, poo and respiration in the form of ammonia. In fish tanks, that builds up quickly and kills em (burns their gills and skin and can suffocate em). Fortunately mother nature comes to the rescue in the form of bacteria which eat ammonia. Problem is, they produce nitrites which as also deadly to fish (competes with
O2 and also suffocates em). Mother nature to the rescue again with another bacteria which eats nitrites and produces nitrates. Nitrates are not a problem in low amounts (under 40
ppm or so) and can easily be removed with water changes
You can also do whats called a "fishless" cycle, which involves adding an ammonia source and waiting for the bacteria to show up. By cycling a tank this way, you add the fish once the ammonia nitrites are zero, and nitrates are showing up; then the fish don't have to be exposed to high amounts of the dangerous nitrogenous wastes.
There is also a 3rd option which is pretty recent. Marineland came out with a product called Bio-Spira. Its the correct notrifying bacteria in a little bag; its added directly with the fish - instant cycled tank! ts the ONLY product I have found which works the way it claims to. Not everyone carries it, but one of our regulars owns a store and he ships *grin*