Aquarium fish that constantly hide among the plants and tank accessories aren't much fun, since the whole point of having an aquarium is to watch and admire all of the members of your ecosystem. But like any other animal, a fish hides because it is scared or uncomfortable with its surroundings. The important consideration is to address whatever it is that is making the fish frightened in the first place. Consider some of the following possibilities.
If a fish has been recently added to the tank, the most likely cause of hiding is that it is simply feeling nervous about its new surroundings. This is particularly true of non-schooling species that often like to claim a specific territory that they can call home. Given a few days, a new fish should become comfortable with its new home and spend more time out and about.
If your fish continues to hide for more than a week, there is another problem at the root. Until you can identify and correct the problem, the new fish will continue to remain hidden.
If fish remain hidden for more than a few days, some of the other fish in the tank may be causing a problem. Sometimes the addition of a new fish will bring out aggressive tendencies in tank-mates that were not formerly bullies. Established fish that have previously staked out a territory may now be feeling the need to defend their territory from the crowding newcomer.
A great way to tone down aggressive behavior by established fish is to rearrange the decor in the aquarium. Once you upend the former territories, all fish must start fresh; there are no more familiar spaces left for one fish to defend. Don't be surprised, however, if all of your fish become afraid of the new surroundings at first and go into hiding. While you will have cured the bully problem, it will take a few days for the new territories to sort themselves out.
Many fish do not feel comfortable unless they have a place or two of their own where they can hide whenever they feel threatened. Providing more hiding places that are always available will often cause timid fish to stay out in view more of the time. Once they feel safe, fish like to look around too!
You can stack rocks to form caves or place pieces of clay pots overturned on the bottom. Add pieces of driftwood with arches or holes, or use any other structures that allow all sizes of fish many more options. If each fish knows it has its own personal hiding places, your happy community will likely become much more active and visible.
Individuals of fish species that normally school with others of their own kind will always hide if kept alone in an aquarium or in a group that is too small. Always keep schooling fish in groups of at least four or five. If they are kept in groups smaller than this, the whole group will find a way to stay hidden most of the time.
Barley, Anthony J., and Ronald M. Coleman. Habitat Structure Directly Affects Aggression In Convict Cichlids Archocentrus Nigrofasciatus. Current Zoology, vol 56, no. 1, 2010, pp. 52-56. Oxford University Press (OUP), doi:10.1093/czoolo/56.1.52