It can be one of the more frustrating things about adding a new fish to your tank. You treat your new baby with care, ensure that it has the right temperature and pH and food and everything else, give it a few days to adapt - and STILL it remains hidden in the back of the tank, behind the rocks, where no one can appreciate your fine new addition.
All fish get a little nervous about new surroundings, and most aren't very happy about the shocks and indignities of travel, so it's normal for a new guy to go into hiding for a day or two as it gets used to its new home. However, if it's still being shy after a few days, that tells you there's something else wrong. Until you figure it out and fix it, your friend isn't going to come out.
The most frequent problem is when your new fish is of a breed that normally travels in schools. Very powerful instincts tell this type of fish that he's not safe if he's alone, or in a group that is too small, and even long familiarity with a tank that's predator-free isn't going to overcome that instinct. With a bigger school, they'll all feel safe and spend a lot more time in the open. If you do your homework before buying, you'll know which breeds you need to buy in groups of four or five or more.
Your next step may seem a little counter-intuitive: add more hiding places to your tank. A nervous fish can be made calmer by providing a safe place he can go. Since even non-territorial fish may each want to have a place of their own to run to when evil threatening kids tap on the glass, the more fish you have, the more nooks and crannies you should provide. More hiding places = more security = more time spent out in the open.
Just decorating the tank in general will make your fish feel less uncomfortable. Prey animals in a nearly empty tank are easily stressed because they feel exposed and vulnerable, which can compromise their immune system and make it harder for them to resist disease and heal from injury. Adding plenty of rock and plant cover, or even neon-pink bubbling treasure chests, will ease the stress, which can help immensely.
Fish that are comfortable in your tank are more likely to display healthy color and natural behavior, and they're going to spend more time out of hiding and be more lively and entertaining to watch.
