Fish can be fickle creatures, and their habits may not always comply with what you want to see in a balanced, lovely fish tank display. Some fish species are naturally reclusive or shy, and others prefer the bottom or top of a tank as a hangout spot. Research fish behaviors before buying to ensure you get the type of fish that will help you create a visually-appealing aquarium.
Fish, especially small fish in an aquarium with larger fish, will naturally congregate around hiding areas. If you have a large concentration of plants or aquarium decorations, like faux rocks castles, on one side of your tank, you may find many fish hanging out there for safety reasons. Try spreading your foliage and decorations throughout the tank and include plants of various height and density to provide an array of potential hiding spots for your fish.
While most quality aquarium heaters are good at disbursing heat in such a way that the water stays at a constant temperature, you may find fish hanging out on one side of the tank rather than another because they prefer the temperature. Purchase two inexpensive submersible thermometers and place them on either side of your tank. Measure the difference in water temperature and adjust or replace your heating mechanism as necessary to create a constant tank temperature.
Depending on the type of pump and filter system you use, your tank may be generating water currents that are easier for fish to navigate in one particular area of the tank. Make sure the pump you’re utilizing is appropriate for your particular size of tank. This will help ensure you don’t send your fish on an underwater jet stream that inadvertently confines them to just one side of the tank.
Some fish will just have a preference for one area of your tank over another. Maybe one section has a thicker algae growth they can feed from, or you may have a larger fish defending a territory on another side of the tank that makes a certain group of smaller fish steer clear. While this doesn't necessarily present a major problem in terms of keeping your fish healthy, you may want to populate your tank with a wider variety of compatible fish types and sizes to encourage full-tank usage.
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