Why did my fish die

Question
I have a 29 gallon live rock and fish aquarium.  I would soon like to move up to reef.  I have a 205 fluval canister filter, Berlin airlift skimmer for up to a 40 gallon tank, powerhead for a 30 gallon tank and a 50/50 light bulb.  My tempature stays stable at 78 degrees, being that I don't believe my tank is fully cycled I still have signs of ammonia, nitrate and nitrite,  My ph is pretty low as well.  Which I have been fighting to move up.  I am very new at this hobby.  I take it very serious and want nothing but the best for my fish.  
    Well to make a long story short the past two water changes I have performed have caused fatality to my tank.  The only one that always survives is my green brittle star.  Everyone seems to be doing fine and then my water gets cloudier than ever and all my fish collapse.
   Please tell me what I am doing wrong.  I want to become great at this and I have realized that if I don't ask questions I will never get there.
    
                      Please Help
                 Princess

Might I add the first water change that caused death was too big.  I know that know.  I did about 50%.  The water got so cloudy I couldn't see through it.  I would wake up and my tank would be cloudy and my fish would be dead.  
 
The last water change I don't understand at all.  Only about 20% water change and the same thing happened.

How come my fish die but my starfish always survives.  I always mix my salt to a point.  I test my water everyday basically.  Please tell me where I went wrong.

Answer
I think it is possible you did not let your tank cycle enough before you added living things. there is a bacteria which flourishes at the right time to take care of the waste produced by our tank inhabitants. I would start over. give your star to your local pet shop to take care of and then go to about.com, and click on hobbies. then click on saltwater aquariums. you will be at the best website to start out properly. take the course - saltwater 101, and you will be on your way to a successful and enjoyable experience. good luck, gerry.