Cycling is basically where your tank builds up and establishes good bacteria that help convert the ammonia produced from fish waste into something less toxic for fish, as a high level of ammonia can have several ramifications on fish such as death. At the start of the cycling process you will see a high spike in the ammonia level in your tank, it will then lower, and then raise again along with nitrite. The good bacteria helps convert the ammonia in nitrite, and then the nitrite into nitrate. Once these two both lower again, you will see a spike in the nitrate. After approx. a month, your ammonia and nitrite levels should read 0, and the nitrate should be slightly elevated to just above 0. Once your tank is able to consistently remain at 0 for your ammonia and nitrite levels, your tank has become established. This will take several months to happen, but generally after one month the tank will be finished cycling, possibility a little bit longer if you a doing a fishless cycle (where you are manually adding in ammonia yourself). The ph level might also rise during cycling, as high ammonia levels can lead to a rise in the ph as well. Fish generally cope well in ph level of 6.5 to 7.5 so your fish should be fine with the changing level of ph, if it does change. The cycling products you are talking about don't cycle your tank for you, cycling is a natural occurrence and what these products do is add in beneficial bacteria to help to the amount of beneficial bacteria your cycling tank is producing. The products you can use for cycling should be used before you add in fish, you should start using it approx. 2 days before adding in fish, as your tank will have no beneficial bacteria at that stage, so by the time you add in fish the cycling product you have added in will have helped your tank to already establish some good beneficial bacteria. I don't know why they would tell you that, because that is complete nonsense. Your tank will always have bad bacteria, and it's usually the good bacteria that dies first. The good bacteria will of course dominate over the bad bacteria, but you need to keep your fish healthy because if they become ill they will immediately attract the bad bacteria.
