Question
I have had a fish for a while and have always filled its fish tank with our tap water from the well. I am moving now and taking the fish. Will the change from well water to city kill it?

Answer
I too have well water.  We had the option to hook up to city water but one of the reasons we said no is because city water has added fluorine and chlorine.  Those are both bad for the fish. But, you can combat them.  Add dechlorinator (also called tap water conditioner) to deactivate chlorine or chloramine. Contact your new water supplier to find out if they use chlorine or chloramine.  If they use chloramine, be sure your tap water conditioner says it deactivated that because it releases not only chlorine but toxic ammonia too.  So, you need something to deal with the ammonia.  Good aeration will drive off fluorine and free chlorine over about a day but it won't do anything with chloramine (you need the dechlorinator with ammonia-lock for that).  So, add tap water conditioner and aerate well.  If you can pre-aerate the water before putting it in the tank, that's good but it's not required if doing less than a 50% water change usually.

The new water will be different than the other water.  The fish need to get used to it.  If you can move some of the old water with the tank, that will help a lot.  Then, you can slowly change it out with new water so as not to shock the fish.

Test the new and the old water for pH, hardness, alkalinity, ammonia, chlorine, nitrite, and nitrate (or at least the first two) to see how they compare.  If there's a large change say from hard water to soft water, the fish have to be eased into that.  Things can be added to the new water at least at first so that it's closer to the old water.

See http://www.fishpondinfo.com/fishcare/water.htm and http://www.fishpondinfo.com/chem.htm for more information.

Good luck!