Which of the following is a sign that fish should not be used?

The presence of dull grey gills and cloudy eyes indicates that the fish is no longer fresh and should not be used. Fresh fish typically has bright red or pink gills, which is a sign of healthy oxygenated blood and freshness. When gills turn dull or grey, it suggests that the fish has started to spoil. Additionally, cloudy eyes are a strong indicator that the fish is not fresh, as they should be clear and bright when the fish is still good to eat. Thus, these characteristics point to a decline in quality and freshness, making the fish unfit for consumption.

In contrast, brightly colored gills and shiny skin are positive indicators of freshness. A firm texture is also a desirable quality in fish, reflecting its freshness and quality. Therefore, it's essential to recognize the signs of spoilage such as dull gills and cloudy eyes to ensure food safety.