Any amount of ammonia and nitrites are considered toxic. A low level of nitrate is acceptable, say below 40. I hope you're doing a fishless cycle! Otherwise you should prepare for fish deaths, too.

You want to monitor your water parameters as you cycle your tank. Every day, take a reading of NH3 - not total ammonia, free ammonia. You'll see it rise pretty high. When it starts dropping, begin testing for nitrite. When ammonia is a 0, you should see nitrites rising. They'll start spiking, just like the ammonia. When nitrites are 0, test for nitrate. You should have a low level of them. That is when you know your tank is fully cycled.

Now, every few days you should test your water after it's cycled just to make sure you haven't had bacteria die off or overloaded your tank. Every week you should change out a percentage of the water. How large is your tank?

Thank you. MY tank is 30 gallons. Does it matter? I have had my fish in the tank for over a week, they seem fine. I bought a used tank and I used some of the old water and did not clean the gravel, so I should have had some of the bacteria in there. I guess I got lucky! I just put ammo-chips in the filter. Will that stop the ammonia? It says on the package it absorbs ammonia.

Well, if the tank that you bought was completely dry when you set it up, then the bacteria would not have survived. You can keep them wet and alive for a few hours at most, but they do need running water and a source of food to survive.

Ammo chips will eat up the ammonia, but they won't last forever. At any rate, in order to cycle the tank it has to go through a spike of ammonia and nitrites, as there are two types of bacteria that eat the two compounds. I'll let someone else who knows more about cycling and chemistry than I do finish off the rest of this.