The most likely reason for the taste in my water is algal bloom.
In the summer, when there is light from the sun and plenty of warmth, algae can bloom on the surface of lakes, reservoirs, and other water sources. While the bacteria is scrubbed from the water during the water treatment process, the moldy taste the algae imparts to the water can be detected at extremely microscopic levels. Some people are especially sensitive to mildew and musty tastes in food (I noticed this well before my husband did, for instance) and it can affect them more.
Here's what my city water website has to say about this problem:
Occasionally Columbus water has an earthy, musty or fishy taste and odor. These seasonal phenomena can be caused by the bi-annual turnover of our city reservoirs, or with the presence of varied algal blooms in the reservoirs or rivers. It is important to note this taste and odor poses no health concern but one of aesthetic quality. - Source
This taste should fade away as the weather gets colder and the algae dies off. But it's distinctly annoying now, and for the first time I'm considering buying a water filter. Until then, I'm drinking my water with lots of lime slices.
What does your water taste like right now? Do you notice seasonal changes in your tap water? Do you filter your water at all?