How do I know when my cycling is done

[hfp75]

You need a source of ammonia..... What did you use to cycle the tank ?

Let's just say a shrimp..... As it floats around in the tank breaking down, every day it's releasing ammonia. So you see you ammonia rise. As bacteria grow and convert ammonia to nitrites your ammonia drops. As bacteria mature and flourish nitrites become nitrates. Every day that shrimp is still rotting and breaking down releasing more ammonia. It is a cycle or chain of events. To have nitrates today, last week there needed to be ammonia in the tank. If you remove the ammonia source you will disrupt the cycle or more specifically, disrupt the chain of the nitrogen cycle.

Long and short when you have a daily source of ammonia (livestock or a rotting shrimp) and your ammonia drops your 1/3 of the way there. When you finally see nitrates rise your good to go. Add a very light bioload (livestock) and remove the rotting shrimp. Give the tank another week to adjust before adding anything else. Then add more, gradually, until your happy with your livestock.

Ammonia - very toxic for everything in the aquarium, if you can measure ammonia it's bad news for anything living in the tank.
Nitrites - moderately toxic, most livestock can handle it. Keep the levels low.
Nitrates - slightly bothersome to livestock. Keep you levels as low as you can. I ran a great lps/softy tank with nitrates at 40.

Long and short, ammonia is bad & the other two are workable.

This is why some people like to start a tank with live rock & not base rock. The cycle is running and your spikes are very muted. It's a quick way through the cycle. Just be sure the live rock is safe or you basically just gave your tank an std.