Kalk
Kalkwasser is cheap and easy, and I am a fan.
But it's not so simple because of the complications with pH....you can do real damage to the tank's chemistry by spiking the pH without even doing anything crazy - just drip too much kalk too quickly, which consumes the available CO2 in the water, which spikes the pH.
That's directly harmful or even lethal to a lot of critters, and, again, extremely bad for the tank's water chemistry.
So plenty of care is called for when using kalkwasser....for these and other reasons.
Most of the folks who advocate it watch their pH via a live reading from an electronic pH meter like this:
While you can take precautions such as slow-dripping, they aren't foolproof precautions....so you'd be wise to do the same and invest in at least a cheapie pH meter.
(Someday I will too.)
There are no such concerns with two-part.....even for DIY as long as you follow "recipe #2".
I advocate using kalkwasser to supplement two-part dosing - mostly to keep the costs of two-part down. (Two part *can* be so cheap that even this use stops making sense....DIY two-part can be *very very* cost effective in the short term.)
Two Part
Two-part is dead-simple.
It's easy as feeding your fish - especially since you already do testing and don't have a problem with it.
I manually dosed my SPS tank for something like six years with far fewer issues than most it seemed. (Not completely without issue.....but that's life in six years!

)
Set up a pair or trio of DIY drippers like these...
...all you need are some wide-mouth plastic containers (or something! these are orange juice containers) and some airline and airline valves.
The valves pictured are the best I've found without spending a LOT more money. The ones that cost more aren't nearly as convenient tho. I spent a little of the money I saved on the valves on some elbows to cut back on crust formation between doses.
(BTW, I just use the Ca dripper for Mg when needed....I never made a third. Might've been better to have three - not sure.)
I did finally get a doser - which has not been an experience without issues - last year, but I still use these drippers to make all the corrections.
I would stop obsessing over using an automated doser - you don't need to make that decision until much later down the road. You can still start using two-part REALLY EASILY right now.
I like and own a Hanna Alk Checker, but from my own experience I can imagine that you might want to also have a Salifert alk kit on hand for when you need to test more frequently. It's cheaper per test and at least for me is MUCH quicker per test.
I do recommend testing AT LEAST daily when you make big changes, or when you're just getting started. You'll know when you can start testing less often because the results will be predictable.
It may have been missed, but Randy told you baking soda early in the comments - I would definitely use baking soda for adjustments like this.
Get some plain calcium chloride (dry or solution) for making calcium adjustments too.
It may be a while before you need a Mg supplement if you're on a good water change schedule with a well-supplemented salt. (Some two-part supplements have some Mg in them, BTW.)
I would also acquire a two part kit from your LFS and start dosing that for maintenance - it shouldn't take much as the commercial two part formulas all tend to be very concentrated.
In theory you can do enough water changes to fix this, but it wouldn't be too practical.
At minimum you'd want to boost alkalinity at least part way up so it's in balance with your other tests.
When you do the fix:
- Run a fresh alkalinity test
- Use the Reef Chemistry Calculator to calculate your correct dose of baking soda.
- Dose half of what it tells you.
- Wait at least an hour or two, or maybe overnight is best.
- Re-test.
- Dose 100% of what the calculator tells you this time.
- Wait at least an hour, or overnight if you can.
- Now re-test all three parameters.
- Decide if you want to do the rest of the "fix" with water changes. It should seem more reasonable now.
Just $0.02 - hope it helps!