What is the difference between hot spring water and regular hot water? In the last arc of KonoSuba season 2, Aqua winds up purifying the hot spring water, turning it into "regular hot water".

What is the difference between hot spring water and "regular hot water"?

The only clue I can see is that it's (hot spring water) animated green instead of clear or bluish clear (regular water).
You might be looking for something that's analogous to 温泉 (onsen) vs 銭湯 (sentō) in Japan.
By definition, onsen is hot spring water which (supposedly) contains minerals and other things that makes the bath more healthy and provide relaxation benefits (animated green in KonoSuba), while sentō is normal heated tap water (animated blue).
Aqua purifying the onsen water would have implied that she had taken out all the &quot;impurities&quot; (i.e. the beneficial minerals) in the water, and leaving it with the inferior &quot;just hot water.&quot;
Regarding animation, it's not always consistent that onsen water is colored green.