Hmmh.

When I was making and selling testkits based on the K2006, I threw out the Taylor calculator to keep people from using them. Why? Because people got too focused on the "numbers" and not focused enough on the pool. The PoolCalculator is nifty in some respects, but in others, it's a even worse version of the Taylor circular slide rule calculator.

Do you have a heater? If not, forget about the LSI . . . it really is not very relevant to you UNLESS you plan to add some calcium (don't!) OR are beginning to show calcium scale at the water line.

but I don't have a aerator.
Yes, you do. It's called "the surface of the pool". It's just a SLOWER aerator than a bubbler or aerated jet. To lower your carbonate alkalinity (the major -- but NOT only -- component of your TA), you just need to lower your pH to 7.0 - 7.2 and keep it there.

By the way, there are several theories about why SWCG's have been associated with upward trends in pool pH. As far as I know, NONE of them have been proven to be correct, so far.

Do you need to drain and replace, to lower CH? Dunno. If you aren't adding calcium, the ONLY reason your calcium level would go up, is if there's a lot of calcium in the fill water, and you are having a lot of evaporation. A good place to start, is by testing the calcium levels in your fill water.

According to the poolcalculator my pH should lower by ~0.8 from half gallon of addition of 31% muriatic acid, but even doing that my pH only went down to 7.6 from 7.8.
Yeah. Well. Don't believe everything you calculate. In particular, it's rather hard to predict pH change unless you have REALLY accurate info about where you are starting from . . . and you don't. There's a pretty good chance your 31% muriatic really WAS 31%. And, if it was, your pool pH decreased EXACTLY as much as CORRECT calculations would have shown IF you'd had ACCURATE numbers for your individual buffers (carbonate, cyanurate, borate?, phosphate?, what?), your gallons, and initial pH.

Again, this is why I prefer the DT method of dosing (Dump a reasonable amount in, then Test to see what happened. Repeat if needed) instead of the TCMD method (Test accurately, Calculate carefully, Measure dose exactly, Dump it in and then be surprised when the results aren't as expected) presumed by the PoolCalculator.

Let me say it again, differently: if the PoolCalculator (or any other pool calculator) were perfect, and was given perfect information, it would ALWAYS precisely predict changes. But, that's not going to happen with real world pools, chemicals, test kits, and pool owners.

So, why don't you do this:

1. Lower your pH to ~7.0. Ignore the PoolCalculator; just add muriatic acid 1/2 gallon at a time, till you get close, and then 1/4 gallon at a time.

2. Test your FILL water CH, and report that.

3. Buy 16 boxes of borax (8 per 10K gallons). Buy 1 gallon of 31% muriatic (2 gallons of 15% ) per 4 boxes of borax, PLUS 1 extra gallon. (So, 5 gallons of 31%). Add 1/4 of your acid. Then add 1/4 of your borax. (So, 4 boxes of borax & 1 gallon of acid). Wait 24 hours, then test. If your pH is low, add 2 boxes of borax. If it's high, add 1/2 gallon of acid. Wait, retest. If your pH is OK (7.0 - 7.8) add the next 1/4 dose. Continue till your done.

4. Order some borates test strips from Amazon.

5. Let us know how it goes.

(By the way, the TCMD method works really, really well for extremely accurate and careful people, like Chem_Geek. The DT method works better for most pool owners. If taking care of your pool is your hobby, use the TCMD method. If you are more interested in USING your pool, use the DT method.)