Here's how I view different test methods
1. Apex/Trident - constant monitoring. Most useful for trends/changes. Not just for alk, but also Ca, Mg, pH, ORP, salinity.
2. Hanna - Periodic spot-checks to verify the Apex/Trident Combo
3. Salifert - Gold standard. Used when "omg what is happening I need to get to the bottom of this" and need to regain trust in my measurements. And a calibrated refractometer for salinity numbers.
I've tested my salifert kits against reference standards and they're always the closest to expected which is why I use them as my baseline. I would not trust the Apex/Trident to be the end-all of testing because there are myriad things that can effect the Trident results that are either harder to control or just don't exist when compared to the Salifert test. Things like: microbubbles, debris in sample or reagent tubes (and other sample/volume related variables), calibration factors, etc.
Going backwards to show my method: I calibrate my Trident using my tank water. I measure a sample with Salifert kits, then immediately run the Trident calibration cycle using the numbers obtained from the Salifert kits. I also determine the calibration offset required by my alk Hanna checker at the same time.
This process was born out of implementing the trident, and my experiences trying to get repeatable measurements aligned with other (more trusted) test methods. Given I've had the Trident less than a month, I still need to determine how often I'm going to perform this calibration process. Needs to be often enough to keep drift in-check, but not so often as to cover up any small changes in tank parameters. And I'm considering ditching the hanna altogether.
Now that that's out of the way -- have you verified your Trident numbers against a standard titration kit? Also, it looks like your alk has been swinging quite a bit as you make changes to try to get to the bottom of this. That's certainly impacting growth. I wouldn't do anything until I verified the Trident with a titration test.
Edit - Just saw that you've tested with other methods and the results are higher than trident. If you're testing your replacement water and it doesn't have high alk, I don't think it makes much sense to switch salts. Feels more like a hail mary based on hope than a data-driven decision, to me.
Something's adding alk to your tank, and I also suspect that it's related to your previous alk issue. Even months later. You're currently at what, 8.6 or so? Take a a few breaths and let things go for a bit and monitor. Step in and make changes only if you start seeing alk burn (or increase nutrients to make the increased alk more palatable to your coral). Right now, you're nowhere close to "too high." It seems like you're stuck in a correction cycle and you might be doing more harm than good by trying to throw everything at the wall and seeing what sticks.
