This has been written about and discussed extensively on these forums and comes up all the time. Most people seriously misunderstand what pH is and how important it is in different contexts. A pH crash is not necessarily a bad thing, in a
fish-only system it could indicate something else bad has happened, the actual pH change itself isn't what hams the fish but most fish keepers do not realise this.
In planted tanks with (often even without) added CO2, large pH swings are commonly experienced at night when the lights go out, and this can happen in nature too. Do not worry about pH when adding CO2 to a soft water (low KH) tank to grow plants, the pH value will drop very low, however it is rather meaningless in this context. If you are really worried, you can add some bicarbonate of soda to the water when doing a water change to boost the KH (which would reduce the pH change). I have very soft tap water too, when I add CO2 I basically just ignore the pH of the water, I have never had an issue with low pH. Low pH being an issue is basically a myth, with regards to planted tanks anyway. Even if it was an issue...it's very easy to solve...
I was concerned about pH swings too many years ago, so added some crushed coral to my substrate when setting up my oldest planted tank, you can add some in a filter bag (oyster shells/chicken grit works equally well apparently). These will slowly dissolve when the water is acidic, gradually releasing buffering KH compounds.
Darrel (member dw1305) has written some very good replies on this such as the following one:
Co2 - sufficient produced overnight? (Follow the links in each post for more details, there's probably a better sticky topic or article on the forums somewhere, I cant find anything atm though).
This is a great thread on the topic too:
https://www.ukaps.org/forum/threads/ph-less-than-6-digital-ph-monitors.18116/#p185647
I would highly recommend the Bazooka Atomiser for inside a tank, this type seems to produce the finest mist. The glass ones look nicer, but I've never had a glass one that works as well as the Bazooka.