Let me add my experience..
I got myself a 3 foot tank and it was cycling time.
Got myself 3 large pieces of Live Rock and some normal sand.
Decided to let my tank cycle for a month.
I didn't put any market prawn in the water though. I spotted 2 of those idiotic hitch-hiker crabs and squashed them. Placed them in a glass in the center of my tank. Removed them once I noticed that they were significantly decomposing.
All this happened till week 2. According to my plan, I had wanted to leave it for one month. Just like that, no fish, nothing.
My dad returns from overseas, gets all excited about the new tank and insists that I put fish in. I knew it would be a bad idea but had to do it.. you know.. my dad.. good God. So the tank claimed the lives of a large Yellow Tang, a large Powder Blue Tang, 2 Watchman Gobies and a false Clown. It killed me knowing that all those lives were wasted. Almost wanted to end the hobby there. Tank was left lifeless for a while.. Dad goes back overseas. Tank finally got to cycle properly. Dad returns. This time Test Kits were smiling at me and everything seemed pristine. Went to get a pair of a pair of maroon clowns, 1 canary wrasse, 2 cleaner wrasses, an Algae Blenny and a large orange starfish. The tank looked awesome for a whole... but I did not notice that the starfish had vanished till about a month too late when the clowns and the wrasses died. Checked the cave under the large piece of live rock. A mushy orange mass was there covered in worms and wierd bugs. Siphoned the whole lot out including the pests. Soon even the Algae Blenny died.
I learnt 2 things from these experiences,
1. Never add Livestock until you are completely sure the tank has cycled.
2. Stock your tank slowly.