What do you mean "high nitrate?" Nitrate rarely kills animals outright. I would look elsewhere for the cause of the deaths, unless you tell me that the nitrate was above 100 ppm. Water changes are generally a good idea, but I would go after the source of the high nitrate.
The tank sounds like it was overstocked. It looks like two dwarf angels, the blenny, the two tangs, two chromis, 4 damsels, a mandarin and a clown in a 54 gallon tank. With that level of crowding, you were on the way to a meltdown. The tangs tend to be more sensitive, and would be the first to go if ammonia were to go up or oxygen down.