Answer: The answer to that is rather simple – the fish is scared or uncomfortable with the surroundings. The more important question is what is making the fish scared in the first place?

New Surroundings
If the fish was recently added to the tank, the most likely cause is that its simply feeling nervous about its new surroundings. Given a few days, the fish should become comfortable with its new home and spend more time out and about.

In the event your fish continues to hide for more than a day or two, there is another problem afoot. Until you correct the problem, the fish will remain hidden.

Schooling Fish
Fish that normally school with a others of its own kind will hide if kept alone, or if kept in a group that is too small. Always keep schooling fish in groups of at least four or five. If they are kept in smaller groups, they may stay hidden most of the time.

Hiding Places
Many fish do not feel comfortable unless they have a place of their own where they can hide when they feel threatened. Odd as it may seem, providing more hiding places will usually cause fish to stay out most of the time.

Stack rocks to form caves, place pieces of clay pots on the bottom, add pieces of driftwood with arches or holes, or use any structures that allow fish to hide. If each fish knows it has its own personal hiding place, it will feel safe and stay out in the open more often.