Cylinder tanks can distort the fish and make viewing strenuous. However, here are two concave tanks of very different size where the concave shape of the window enhanced the viewing.
At the Long Island Aquarium, the main shark tank is a large fiberglass tank with about 30 feet (ca. 9 meters) diameter. The windows are curved out, bringing the animals closer to the viewer. The windows are only 8' wide, that creates an intimated viewing, excellent for a family to observe and share their experience in a more private atmosphere. Compared to larger viewing windows, it made a nice contrast. Because the curve was so slight, the distortion was less than that of your average cylinder tank.
Also, noteworthy: The Long Island Aquarium doesn't have an acrylic tunnel — I didn't miss it.
The second tank (at the 1min4second) shows a hemisphere acrylic window of about 3 feet (ca. 91 cm) that leads to a very strong magnification. It made the tiny animals pop out and also made an interesting viewing. The distortion was not hindering the view but making it great.