Hi navy. In order to kick start cycling, a food source needs to be present for bacteria to start working.
to put it very simply, there are three kinds of bacteria. I wont go into scientific details here to prevent confusion.
Bacteria A consumes ammonia to form nitrites.
Bacteria B consumes Nitrites to form nitrates.
Bacteria C consumes nitrates to form nitrogen gas. however, bacteria C cannot tolerate oxygen, and thus can only live in places where there is no oxygen, like deep within the sand or deep in liverock. unfortunately bacteria C is very difficult to cultivate, and that's why nitrates are the end of the breakdown process in most tanks. Rarely does it get converted to nitrogen gas.
To begin cycling, bacteria A needs ammonia to grow. That is why people put market prawn. As the prawn decays, ammonia is produced, and bacteria A can convert it to nitrites. This is where your question is answered. Instead of prawn, some people put damsel fish. Fish produce waste, and the waste will become ammonia. But as you can see, this is unnecessary and cruel.
As the ammonia builds up, more bacteria A grows and then more ammonia is converted to nitrite.
As nitrite builds up, bacteria B starts converting it to nitrates. and then, more bacteria B starts producing.
Finally, a stable healthy population of both bacteria A and B will be achieved. And they will convert these harmful chemicals to safe nitrate for you to remove via water change or other nutrient export.
Nitrates are safe but are toxic in very very high amounts, in the hundreds of PPM. so don't worry too much about it unless you are keeping SPS.
Hope this helps!!
This is the summarized, version of my original article above. Hope it's easy to understand and i hope it helps you