NWW, as a kid I worked in the local pet store which was owned by an Ichthyologist. For new tanks he started people out with one or two goldfish or mollies and he told them not to expect these fish to survive. The purpose of these fish was to "break in" the tank. It sounds like your tank hasn't developed the beneficial bacteria that convert toxic fish waste into safe compounds. Changing the water will help the fish somewhat, but changing half of the water can also be another source of stress. What type of filter do you have?