Tea is almost all water, so the water decides more about the cup than most people expect. Hard water dulls aroma and leaves a film on the surface. Very soft water can taste thin.
A simple filter jug or a filter on the boiler line is usually enough to make a clear difference. Taste the same tea brewed with filtered and unfiltered water side by side.
Temperature matters as much: boiling water is right for black teas and most herbal blends, but too hot for greens and many oolongs.
