Ascorbic acid would be better to use for removing metal stains than citric acid and yes, it's like magic. See
this link for more info on that.
As you have noted, a high TA will result in a faster pH rise and this is accelerated by aeration such as the water features you mentioned. You should follow the procedure in
this post to lower the TA. Do not use the slug method. It creates a VERY low pH locally and that can damage pool surfaces. The amount that TA gets lowered is solely based on the amount of acid that you add. The method of addition has nothing to do with that, but it does affect how quickly and efficiently and safely you can lower the TA. Acid lowers both TA and pH while outgassing of carbon dioxide raises the pH with no change in TA. So a combination of acid addition with aeration will lower the TA and doing this at a lower pH makes the process go faster. In 15,000 gallons, 4.8 cups of full-strength Muriatic Acid (31.45% Hydrochloric Acid) lowers the TA by 10 ppm. The only reason you don't add all the acid to lower the TA at once is that the pH would get too low and could damage pool surfaces and equipment.
The other problem with high TA is that it raises the calcite saturation index so can lead to scaling. This is especially true if the pH or Calcium Hardness (CH) are high. So continued use of Cal-Hypo that you are using will raise the CH and the water can become cloudy or scale can develop. It would be better for you to use chlorinating liquid or 6% unscented bleach if your saturation index is too high. You can use
The Pool Calculator for calculating dosages and for the saturation index. You need a good test kit: either the Taylor K-2006 you can get at a good online price
here or the TF100 kit from tftestkits.net
here with the latter kit having 36% more volume of reagents so is less expensive per test.
Does your Metal Out product list any ingredients or does it have a manufacturer name on it? If it's EDTA, then this tends to break down faster and consume chlorine more while HEDP will not.
Also, are you using any Cyanuric Acid (CYA) in your pool? You said you are avoiding stabilized chlorine to avoid CYA buildup, but did you ever use any so you have at least some CYA in your pool? Even though it is indoors, a small amount of CYA (say, 20-30 ppm) will reduce chlorine's strength so that it isn't "too" strong. This will be less harsh on skin, hair, swimsuits and equipment. It should also lower the rate of some disinfection by-products, mostly nitrogen trichloride. You don't want too much CYA, however, because it would slow down oxidation too much if it were too high.
Richard
My question, other that causing the PH to creep up, what other adverse condition does a high TA cause? I have an indoor 15,000 gal fiberglass pool with the TA at 510 and the PH is 7.4. I have aeration built into the pool (slide, waterfall, jetted seating) so I'll try the aeration/acid method. My pool shop suggested the "slug" method.
