Yeah most often calcium carbonate is added
or you end up like flint Michigan. Acidic water and pipes are a no no.
When co2 is dissolved in water it creates carbonic
acid and that drives the ph down. In open water it's not an issue as it will off
gas quite easily but under pressure is a different story.
That's exactly how I used to feed my aquatic plants co2. You can't just push co2 through an airstone as it not very effective so I built a co2 reactor to dissovle it and kept surface agitation on the low side
The lime is to act as a buffer for the carbonic
acid created after from the break down of organics caused by chlorine