When you spot a dead leaf the best thing to do is take it off asap! If it's a disease(bacteria) causing them to die, it will help slow the progression. And you cant bring back to life the dead anyway. And water does not need to be at absolute zero between 1 and 10 ppm's is fine. And try to keep the ph of the soil between 5.8 and 6.4. But some sativas may like a ph slightly higher than that depending on the strain.