hth Spa 86226 pH Senker Ausgleichspflege für Spa und Whirlpool, 1,1 kg

Q: Does this lower Alkalinity as well??

If you lower a pH of 8.3 to the recommended level using muriatic acid, the total alkalinity will also reduce but will still remain on the higher side. Likewise, if you lower alkalinity to the recommended level (e.g., from 300 ppm to 80 ppm), the pH level drops way below the recommended level. So according to this yes.