Since your water is soft, it may also have a low alkalinity (low buffering capacity) which may make more regular water changes neseccary. You could call your water provider to find out those parameters, or it may state them on your water bill.
Low buffering capacity means its easier for uric acid and other metalbolism by-products and acids to build up, to ruin water quality, and can create high nitrates, much faster between water changes. This is why any text book standard water change routine is nebulous, because different cities, different states may all have different water parameters dictating how many, and how frequent water changes may needed to maintain a tanks water quality, which the different species of fish may need.