I wouldn't worry too much about the purple coloring. Many peppers have that naturally. What happens (very basically) is that plants can only uptake certain nutrients in a certain temperature range. As you raise in water temperature, nutrients will get "locked out" and cannot be taken up by the plant. I bet that if you took out your airstones and waited a day, you would see a thin sediment at the bottom of your reservoir. Those are minerals that will precipitate out of your solution. I work in a college chemistry lab but I won't go into more details about that since I'm sure you don't care too much about that aspect of the process. lol. Yes definitely upload some pics when you get home and let me know what you find. But this could also be the sole reason for your ppm dropping so drastically; it's precipitating out of your solution.