Saltwater Tank, Delicate fish dying?

Question
I've had my 75 gallon setup now for about 2 months and cycled it with 2 hermits and a few damsels.  I slowly added two trigger fish, and a clown.  One of the triggers died about a week later.  Month later I added a wrasse, an angel and an emperess.  The wrasse died about a week later, then the other trigger followed just today. (No sign of ick)  The damsels (4) are doing well and so is the clown.  The emperess and Angel are doing good too from what I can tell.  I was told having a 75 gallon tank was easier to keep up because of the size but so far I've lost 3 fish and am getting very discouraged.  I will say I don't have a heater in the tank and the water temperature is probably about 70 degrees.  Was this too cold for the triggers and wrasse?  

Answer
Hello Ivan,

    I think you hit the nail on the head. The average temperature for saltwater fish is 78 degrees. It is amazing to me any of your fish are surviving. Get a heater asap and have your water analyzed at the store where you buy your fish. There are several other pieces of equipment you will probably need beides the heater. One important addition would be a protein skimmer. This takes pollutants out of the water. Please let me know how you are doing after you get your heater. Also are you familiar with, and how often to do water changes, and how much to change? gerry.