Do I need to treat spring water?
You shouldn't have to since it shouldn't have in it.
I think it varies, and it ultimately depends on whether "spring water" is just marketing liberty or whether it's actually sourced from a spring.
I know a lot of bottled "spring water" in Australia tends to just be sourced from an urban water system: it's got chlorines, chloramines and all that in it. Water conditioner is cheap enough to use if unsure.
Some bottled water has a mineral analysis on the label. Otherwise you can do a GH test, but this doesn't necessarily tell you what sorts of dissolved minerals are in it. Usually in freshwater most of GH is some combination of dissolved calcium and magnesium.
I will add conditioner just incase. Thank youI would say, no, you don't need a water conditioner with this water. I prefer using Prime as my go to water conditioner. Even if I didn't have chlorine/chloramines in my water I would still use it because it also neutralizes small amounts of ammonia and/or nitrites. I consider it a cheap insurance.