When you first set up an aquarium you basically have a tank full of water that will not be able to sustain life for very long.  In order for your fish to survive you have got to cultivate a bacteria colony inside your filtration system.  This bacteria colony will consume and remove all the dangerous toxins that your fish produce, therefore keeping them alive and healthy. This initial process is commonly known as "cycling" an aquarium.  To some people this might sound rather daunting, however it's quite the opposite actually.  Cycling a brand-new aquarium can take several weeks, up to 2 months depending on the size of the tank.  Until the process of cycling an aquarium is complete, your fish will be at risk from toxic poisoning.  Therefore it is vitally important that you understand the importance of cycling your aquarium properly.  Further reading can be found by clicking the link below How to cycle an aquarium...