Tap water is fine if you PH it. PH is very important. The PH of your water should be between 5.5 and 6.5

I would suggest going to the pet store and buy a PH testing kit. You will find them in the aquarium section for like 10 bucks. It also includes ph up and down to adjust the water.

Use that ph tester to test the ph of your tap water, then adjust it accordingly.

Heres the link

Pet Food, Supplies & Accessories at PETCO