Also are you familiar with the nitrogen cycle and properly cycling new tanks before adding fish? If not read about it here: http://www.aquariacentral.com/forums/showthread.php?t=84598
Did you cycle your tank? If not, that could easily be the problem. If you haven't already, you should get a liquid test kit, test your ammonia / nitrite / nitrate levels and then take action as needed depending on where your cycle is at.