Spotted Stamps After Soaking

I have been soaking Belgian stamps in tap water and when I pulled some out of their bath I noticed that a couple had some spots on the back and I wanted to make sure that the stamp isn't damaged by the tap water.

Yikes, what exactly did you soak them in?

Just plain tap water, I am planning on soaking a batch in distilled water and seeing if that makes a difference

Probably ink or similar from the surface to which they previously adhered.

Agree.
Opinion:
Forget distilled water, just plain clean water from the tap, (cold) allow 10-30 mins, then rinse and remove any obvious gunky mucilage.

Self adhesives need another method.

Wow! It does look like someones ink pen exploded or splattered or the stamp was used as a blotter. As the others above state, tap water is fine.

As for Canadian self adhesive stamps only, a special gum was manufactured for them, allowing them to be seperated from the envelope and adhesive. Just require a little more time, and patience in lukewarm water to seperate cleanly with no tears or thinning.