On my 90 I just put one large raw Prawn in last night, for me that should be plenty, for a 180 I would say 2 or 3 would be right. I hung mine by using some fishing line in the middle of the tank so that it was in the path of the spraybar that is in the top/back of my tank. After a couple of weeks there should be plenty of first stage bacteria that should be well established, and should be able to keep up with the amount of ammonia created by the dead prawn and changing it into nitrites. After 3-4 weeks your second stage bacteria will begin to proccess the nitrites into nitrates, and will build quickly, then all of the sudden your nitrites will drop and you should be able to check this with your test kits, After that you should have 0 ammonia, 0 nitrites, and some nitrates. If you are still having a problem with the ammonia, and nitrites its time to take whatever is left of the prawn out. Some take them out early some let em rot. This is where you should add your first fish to keep the cycle going, that is the nitrafication cycle, that way your bacteria have somethng to process and stay healthy and can handle the bio load, as you slowly over the next few weeks add more fish, the bacteria will grow to keep up with that bio load.
Thomas