In my experience, I was once an extremely uneducated and unprepared aquarist who decided to get three glofish for a ten gallon freshwater tank. They were dead within hours because ammonia spiked so much. Don't worry, I know much better now! poor fish...
Basically we cycle our tanks so we can prevent ammonia and nitrite spikes and establish a biological filter, thus keeping our fish alive. If you add fish before you cycle the tank, you'll be doing a fish-in cycle, in which ammonia is produced via fish waste or corpse (as stated above). OR if you add them early you'll get what I've started referring to as the glofish result: all dead
So don't neglect your cycle!