Hello,
You have a pretty small tank, so any change will be extreme compared to my 240 or even a 125. Knowing this, leave the baking soda where it belongs the fridge. You do not have the water capacity to make a mistake. I suggest 10% water changes every week, like clock work. Larger tanks can go longer, but your size, weekly no exceptions. I dose my tank every tues, thurs, and Saturday. In respect, I don’t test my own water, my lfs does. After the test is complete then I will dose what I need. My method is yes unorthodoxed, but it works and they have better test kits and do it for free

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For your tank being three years old, it gives me concern that a mature tank needs this kind of dosing. My tank is new (I had one before it and just upgraded it), (7 months) where it’s still in that stage of it could swing and have a bloom etc. yet I have only ever dosed alk and calcium. I try to keep at 9 and within two days if I don’t dose I will be at mid 7. The same is with calcium I stay steady between 440-450. If I don’t dose I’m down to 330 in two days. Sometimes I have to skip calcium, I have been as high as 500, but I didn’t dose it until I was back in the safe zone. But I also test three times a week, and I dose the same amount every time.
You mentioned that the stunner and sps were not well. (If memory serves me right, I may have misread FYI but believe I did), if so this proves your alk swing and other parameters are out of whack.