Dechlorinater is also somtimes called "water conditioner" too; if you don't add it to your tank your tanks ecosystem(used to help break down waste in the tank) cannot develop because the chlorine in tap water(put there by water companys to help kill desease that may be passed through water) also kills your tank ecosystem good bacteria. If you don't have this all kinds of water quality problems arise in your tank which usually lead to death and desease in your tank.