As others have already said, there is no need to add any pH control to your water. The tap water pH for most fish would be just fine. The problem with adjusting pH is that it will start to vary with time and as the chemicals you use get used up the pH will drift around. If you have a natural tap water pH anywhere from about 6.5 to 8.0, most fish will be just fine. One piece of bad information that you had is that pH indicates hardness. It does not. PH only indicates whether the water is acidic, basic or neutral (an excess of H+ or of OH- ions). Hardness is a measure of the concentration of calcium and magnesium ions in the water and, since those chemicals are often present in water that has passed through a lot of rock, pH is a secondary way of judging hardness. Unfortunately, hard water can have almost any pH but people react to pH as if it indicated hardness. Tank water being held at 7.0 by adding chemical buffers may be very hard or very soft before you add the buffer but is undoubtedly higher in mineral content after the buffer is added. In general, fish reputed to prefer soft water really prefer water low in minerals and fish that like hard water really doing better with high mineral content water.
It sounds like you may have cured your original problem of fish being affected by metals in the water but there is no reason to change the type of fish that you keep unless you actually know what kind of water you are working with.