Tonic Water Off-Flavor

Has anyone here ever come across an off-flavor that I can only describe tasting like tonic water (like when you get a fountain soda that doesn't have enough syrup in it).

I did my first partial mash, and the wort had this taste to it as soon as I was done the boil. It has carried through 4 weeks in primary and secondary as well.

I usually use bottled spring water for my brews, the only difference this time was that i used approx 1 gallon of tap water for the mash.

Any ideas?

Hard to guess, but maybe low sodium? Or other minerals.

At what temperatures did you mash and sparge?

I mashed at approx 154 and it probably dropped to around 150 by the time it was done (pot was a little thin). Sparge i believe was at 175.