Fish keep dying.

Newbie help please! I have a 60 gallon reef tank and have had great success with the corals but can't seem to keep fish. Its been up for 4 months and the water is testing perfectly at home and at my LFS. The first round of fish seemed to do fine (2 clowns and a tang) and all at once they all died. Did a series of water changes and started stocking the tank over a couple weeks again (tang, 2 clowns again). They were fine again for 3 weeks and 2 died, followed by the last one 2 weeks later. The corals are looking good, and the snails, crabs and shrimp are fine (all from the original setup). The tang did look like he may have had ick (he developed white spots right before he died, had labored breathing), so I'm treating the water daily for that now.

I do bi-weekly water changes of 10 gallons (red sea water from LFS), topping off with grocery store bought distilled water. Hang on the back filter, heater, LED lights on a timer. Water temp is at 79.

Any help/ideas would be much appreciated!!!