Fish spend most all of their time searching for food, except at certain times of year when their minds turn to reproduction.
Oceanic fish are real wanderers and travel 1000s of miles in their lives, constantly on the move looking for food (like wildebeest in Africa)
Fish behaviour outside the tropics can be quite seasonal – for example herring spend the winters in the north atlantic much deeper down to shelter from storms and live nearer the surface during the summer
Like James said, fish spend most of their time trying to find food and to not get eaten by other bigger things 🙂
But when they ARE reproducing, some fish stay at their nests or with their young for days or weeks at a time without eating much. They put most of their energy into protecting their nest and their babies from predators.