When I was tuning my CO2 injection, pH was tested with a calibrated benchtop Hanna I borrowed from my lab. I trust its accuracy and I haven't changed the metering valve since then. A pocket pH meter I have at home was surprisingly close to the Hanna after calibration. I don't really use the API kit except for quick checks.
KH was measured in a way similar to how you do it -- I used the API kit with 40ml of tank water vs the 5ml suggested by the kit. Color changed between 17 and 18 drops, so I estimate KH of around 2.125-2.25. I could probably get extremely accurate results if I do a proper titration in the lab...but I think I'm close enough and I would likely run out of the test solution.
Circulation is good, I don't see any dead spots and some of the affected Ludwigia, Rotala, and Lobelia are in the highest flow areas of the tank.
I've read too many threads where the deficiency was CO2 so I wanted to make sure I could rule that out, hence my use of the Hanna meter and using 8x the water volume for the KH titration. I have a lot of rippling of the water surface -- a jet lily pipe makes it look like a fan is blowing the surface. I figure I can push CO2 levels to 40ppm or more as long as enough oxygen is dissolving in the water column. My surface skimmer also should help to increase oxygenation so I don't think my livestock will suffer if I push it a bit higher.
That’s all good. We so often see CO2 assumptions based upon flawed testing, but you’ve gone the extra mile and your description of circulation seems good as well. As you said, might still be worth trying to push CO2 higher to see if it helps on the curling.
Another thought on pH: while you may be testing well, it may be that the pH drop is not entirely attributable to CO2. I’m not familiar with Contrasoil, but some substrates buffer pH down into the 6’s. So, if you can determine if this is the case, pushing the CO2 higher might be productive.
I'm definitely open to messing around with different forms of Iron, especially EDDHA as it's the most stable. I was hesitant to try it due to concern over wine-colored water though.
I didn’t see the EDDHA suggestion and was focused on the gluconate. I have tried EDDHA and it did turn my tank water slightly reddish, but don’t recall dosing levels.
You may also want to consider the possibility of simply too much ‘stuff’ in your tank. I have recently come to the conclusion that loading a tank to ensure non-limiting levels of nutrients does not work as well, in my tank, as trying to balance nutrients at low, but non-limiting, levels. Others, here on TPT, push nutrients very high and see negative results if they try to pull back on their levels. So many variables involved that it is hard to tell what balance of things will work in any individual tank. All-in-all, I am in the camp that believes that there is an inter-relationship among most nutrients, but there is a tolerance range within these ratios, e.g.; push micros high and you may need more macros and then there is a ratio balance just within the macro and micro world.
In your case, it seems like you are adding a lot of stuff: Contrasoil may be adding/storing nutrients, root tabs (which aren’t necessary) are adding, plus you are dosing to the water column. I’d start by determining if the Contrasoil, alone, is supplying enough of some of what you are adding.
Since you seem to have a good grasp on the basics, and then some, you might want to cruise through this thread: https://www.plantedtank.net/forums/1...ix-thread.html
It is, more or less, based upon high micro levels, but it also contains a great deal of discussion on nutrient balancing as well as levels. Some approaches may work for you. As I said; personally, I have moved to lower levels of micros and approach my macros by attempting to balance the ionic mix. In my tank, this has caused a pop in colors and reduced GDA and BBA.
Afterthought: what is your TDS?