Using live fish to cycle a tank is not humane. You are intentionally harming the fish and causing it to suffer. Even low amounts of ammonia burn the gills of fish which affects the amount of oxygen they can take in. They tend to die due to lack of oxygen. Suffocation. It is a slow death that causes the fish to suffer.
If you do cycle tanks w/fish, the proper way to do it is to monitor the ammonia levels to make sure they don't get high, keep on top of testing of ammonia levels and keep on top of water changes (sometimes water changes need to be done daily to keep the ammonia levels low). Low amount of feeding helps. This type of tank cycling can take longer b/c it will take more time to build up the bacteria in the tank that breaks down ammonia, nitrite and nitrate. All these levels need to stay low for the safety of the fish in the tank.
It is much faster to use some other ammonia source in a fish-less tank for the cycle. That way you can create high levels of ammonia which in turn allows for larger amounts of bacteria to grow in a shorter amount of time.
It is too bad that lfs are still giving out advice to cycle tanks w/fish w/o the additional info on how to keep the fish safe and healthy in the process. The fish is a living creature and should not be subjected to this type of treatment or considered disposable.
Off my
now. This rant is not directed at anyone here. I'm just sick and tired that this advice is still given out.