distilled, but test that too, it could have some phosphates(so i've heard)
as for spring, yes, do be cautious, often it has phos., trates or amm., PLUS the fact that what makes it a spring gives it a very high chance of having MINERALS, some of which you may desire in a tank, but iron and other metals, you do not want,and they could be in it too
oh, and often anymore, spring water isn't spring water, if youread the label dclosely, it is bottled from a treatment company much like a town plant, not taken directly form a spring, this is why aquafina is so $$, it is run through a RO unit, first