Why neutralize the pH? Swords love 8.3. Much better than 7.0. On finding dead fish, check ammonia, nitrite and nitrate. The only reason to check pH is that a sudden drop can indicate really high nitrate. Most common cause of livebearer death is "female problems", nothing you can do. Green specks are algae, another indicator of high nitrate. A water change would be much more effective than adding acid.
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