Co2 is the main problem. You trying to compensate by overdosing is another.
It may very well be, but I dont see how I can be Co2 deficient now when up until two months ago, I didnt have any - same plants, same light.
This is not a high-light driven tank, it's in the low end of medium at best. It's the same light I had for almost 5 months with nothing but Excel and Flourish comp. Everything grew fine, except as plant mass effectively tripled, I began to notice what looked like a macro deficiency.
Enter Seachem liquid N P K Fe. These helped a lot, for a brief period of time, @ around 1.5 "beginner dose."
Couple weeks later, added DIY Co2. (yes I know the consensus, not good for a tank this size) My system is very sound though, and any is better than none - as long as it is consistent - right? Im doing thee 2 liter bottles staggered 1 week apart. There's no up and downs. Degas and PH/KH tests indicate in the 20-25 ppm range, although I suspect it's not that high. Drop checker is green though so it may be. Here's a build thread
with full details. Regardless, plants absolutely blew up, changed color, etc. Only thing after a few weeks the macro deficiency got worse and worse. Which came as no surprise after the Co2 accelerated growth. Those seachem liquids are incredibly weak, esp at the dose I was using.
Which brings us to one week ago, dry ferts arrived and I started EI routine based on 55 gal levels. My tank is 75 gal btw, so obviously Im not trying to "compensate by overdosing", as you put it.
If there was a deficiency, then things should have gotten better. I believe the slight potassium symptoms are secondary and unrelated. The negative response was so dramatic, and happened so fast is what makes me suspect toxicity. If low Co2 was the problem, I doubt it'd make leaves wither and start dropping within a matter of days. Would it?
Regardless, Im going to do a full 75 gallon dose for a week or two and see what happens. Things will either get better or worse.
Thanks for the suggestions.