Ok, I don't have a solution for you just yet but there are a few things raising eyebrows. I usually see patchwork tissue loss when the nutrients are very low, especially with very low phosphate. That 0 ppm reading suggests that might be the case but I find it strange that you are getting this when you haven't changed the media recently. It's usually the drop that sets them off; however, if you've had extremely low phosphate for a while you could have been keeping them on a knife's edge and something tipped them over the edge.
The other is the doser. The timing of putting the doser online and the recession is suspicious. What's your dosing schedule like? How much of each are you adding at each interval and how frequently are you dosing? It might be causing fluctuations, especially in your pH. What were you doing for calc/alk maintenance before the doser?
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