Tap water can contain heavy metals, nitrates, phosphates and a bunch of other things that will effect the tank.
is better then tap water but can also contain phosphates and nitrates, RO
is the best water you can get for your tank. The filters can be bought for around $100 and is a great investment if your getting into the hobby.
Using bad water can effect coral growth and will promote algae growth.
Read, read and read some more before you jump into the hobby. A little knowledge goes a long way in this hobby.
Also cutting corners in the beginning will more then likely cost you more money in the long run, oh!! and more headaches..
what type of tank are you thinking about? Live rock?
Lights, sump, skimmer?? What type of fish do you want to keep?? Any info will get a better response.