It is impossible to keep fish without maintaining the correct water conditions. In order to effectively care for your fish this has to be done indirectly by ensuring the water conditions are correct. The majority of coldwater fish are happy with most conditions of tap water and will thrive on a natural Ph 7.0 (acidity or alkalinity) however most fish will acclimatise and live happily between 6.5 - 8.0, there is something wrong if your tap water excides theses parameters. The general hardness of the water is the amount of calcium and magnesium dissolved in the water and that would have to be epically high to influence the wellbeing of the fish. It will almost certainly be unnecessary to alter the pH or the hardness of the water for coldwater fish but it is very important to initially condition the water from chlorines and chloramines to be safe for fish, preferably a couple of days before they are introduced. Aluminium chlorines and chloramines are introduced to tap water to sterilise it from bacteria to ensure it is safe for human consumption.
Fish are dependent on bacteria to develop in the filter and maintain good water quality, so initially conditioning with a decolourisation fluid is a must. It is highly recommended to do a small partial water change weekly to help freshen up the water and dilute any pollutants that are present in the aquarium. If it can't be done weekly then a slightly larger (around 25%) should be done fortnightly. Water conditioner should be added to the aquarium during this change to ensure the tap water doesn't damage the bacterial levels in the filters. Aquarium lights should not be on for more than 10 hours a day as this can stress the fish and also encourage algae growth.