I used to used reverse osmosis water when I had a marine aquarium and would recommend this. Your fish will be healthier and you will suffer less algae growth. You can buy RO water from all aquatic retailers (about �4 for about 25 litres) or get an RO unit installed. Having said that, freshwater fish are generally more hardy than marine, so tap water is ok but should be treated first with an appropriate treatment as BOO says. As you were told, never clean a biological filter in tap water: rinse it in old tank water, otherwise you will kill all the beneficial bacterial in the filter.