There's a section called Tropical Fish Emergencies and you'll get more replies if you post this there.
Can you give us a little more information?
What is the ammonia, nitrite, nitrate and pH?
What other kind of fish are in the tank?
How big is the tank and how long has it been set up?
Did the fish that died show any symptoms of illness?
Hi, and welcome to the forum. Hope we can help you with your problems and keep the rest of your fish alive. Post your questions in the right place and you will get a lot of help.
Sasha is right, having a perfect temperature is far from having a perfect tank. There's many other factors you must consider, most contribute to the cycling process which every tank needs to go through in order to keep fish alive in it safely. Read more on cycling your tank here: http/www.fishforums.net/index.php?showtopic=10099
Please post test results in ammonia,nitrite,nitrate,and ph, if you haven't got these test kits i would recommend a water change, what size tank is it and how many fish and which type.