Fish may stay on a particularity side or corner of an aquarium for any number of reasons. Sometimes, this may signify stress or fear. Other times, staking out a particular side of an aquarium can represent a perfectly normal behavior for certain species of fish. You have to understand your species' normal behavior to determine if the sticking to one region of an aquarium means trouble or not.
While fish lack the intelligence of mammal pets of dogs or cats, they tend to put what brainpower they have toward food. With this in mind, most fish will learn to hang out where you feed them. If you always feed fish on one side of the aquarium, fish may learn to stake out this side of the aquarium. If you use any kind of feeding dish, as you would with seahorses, you may find fish hanging out inside the feeding dish.
Fish may hide in a corner or on one side of an aquarium when they face stress. Many different factors can stress out an aquarium fish. Not enough hiding places, the wrong water temperature or boisterous tank mates can all stress out fish. Stress can weaken fish and leave them vulnerable to lethal diseases. You need to ferret out the cause of the stress and eliminate it to ensure your fishes' continued health.
Some fish may carve out your aquarium into separate territories. If a fish stays on one side of the aquarium, it may have "claimed" that particular side of the aquarium. Territorial fish may bully each other to establish a hierarchy. However, if the fish have established boundaries and stopped beating on each other, fish sticking to their own sides of the aquarium does not nessisarily mean any kind of trouble. Look out for injuries and fighting among your fish. If necessary, add more hiding spots so your fish can avoid each other.
Fish may also habitually hang out in different levels of the aquarium. For example, many catfish tend to spend most of their time near the bottom of the aquarium. Conversely, fish like freshwater hatchetfish prefer to swim near the water's surface. Generally, territorial fish tend to live near the bottom of the aquarium. When planning your aquarium, you should shoot for a balance of fish that prefer the top, middle and bottom levels of the tank. A fish's preferred water level can help you evaluate a fish's health and stress level. For example, if a fish normally lives at the water's surface and suddenly hides in a corner near the bottom, your fish is either stressed or diseased.
Some fish prefer light more than others. If one side of your aquarium gets more sun than the other, fish might stake out a hangout based on their lighting preferences.