Tap or Dechlorinated water

[Malum Argenteum]

trace pesticide residue in water (nasty!) and i know fish are very sensitive to it (the EPA tracks it pretty carefully) so much that it can ruin their development. I imagine a snake would be similarly sensitive.

Aquatic animals are distinctly sensitive to chemical pollution, and especially pesticides. Reptiles are not, generally.

Compared toxicity of chemicals to reptiles and other vertebrates

If tap water is safe for human consumption, it is safe for snake consumption. I use RO water usually, simply because that's what I use for a lot of purposes in the reptile room. The 'no minerals' issue isn't much of a concern for snakes, since unlike humans snakes drink very little water (and also unlike most humans snakes have a pretty decent diet) and so don't depend on water for a mineral source.