Promoting Gas Exchange
Water movement allows for the gas exchange to occur by moving dissolved gasses out of areas that may have become stagnant, or areas that may be blocked by any water current from decorations or tank layout. Many various chemicals and gasses such as ammonia, sulfur, and nitrogen may be building up in a specific dead spot as all of the species' waste is collected in a specific area, or debris from the tank such as dead leaves, pieces of wood, etc. might be rotting and creating an invisible pocket of toxic gasses. Having a water current through the whole tank will prevent these chemicals in gasses from building up in any specific spot due to the mixing of the water not only on the surface but also near the bottom. One method of checking to see how strong a water current is in your given tank is to use food and see where it follows before it ends up on the bottom of the substrate. If you notice a dead spot where the food tends to collect (or any debris over time starts to collect), then it might be wise to purchase a filter, power header, or another piece of equipment to create water movement in that area.
Supplemental Water Movement
Proper water movement enhances filtration by transporting more debris and waste aspects into the filter intake. Mechanical, chemical, and biological filtration all benefit from proper water movement inside of any tank.
Slow Water Movement
If you own any species that may not prefer a fast current through their tank, or if you have a small tank and adding anything would create too much current, we recommend the following to limit any dead spots around the aquarium. If you can add in any filter, we would recommend possibly buying another Hang-On-Back filter, Internal Filter, or even adding in a Sponge Filter to promote movement and create a new current. Not only will these move water just on the surface, but they also will push water around near them into deeper levels which should create a new water current to move that dead spot around (or completely remove it). If another filter wouldn't work, or you have some fry and can't afford for them to be pushed into a filter from the water current, add in an airstone such as a bubble wand that can create a new current to move the water around from the bottom to the top. They do this by pushing water around with the small bubbles and even will create water movement at the surface as the bubbles pop over time.
Fast Water Movement
If you own any species that prefer a faster current or have a large aquarium in which the slow water movement methods don't make much sense, we would recommend adding in a powerhead (or a wave maker in a saltwater aquarium if you currently do not have one). What is the difference between a wave marker and a powerhead? Powerheads will specifically push water out through a nozzle, where a wave maker creates a simulation of waves where it will push and pull water to make waves in your tank. These tend to push water out much quicker than they come out from a filter and have various nozzles that can push them out through a fan, or a direct nozzle if you want to specifically target an area with water. Dependent on what species are contained within your aquarium, it is highly recommended to purchase one that contains some sort of filter/mesh by its suction area to avoid any fish or debris being sucked in and causing damage to themselves or to the powerhead/wave marker.
Since fish species tend to be in a tank that is far smaller than their natural habitat, this can cause them to take more resting periods or move more slowly within an aquarium unless they are stressed, or sense any form of danger (such as when you try to catch them inside of a fish net). By swimming against or through moving water, fish receive the correct level of physical activity necessary for proper growth and muscle development. The resistance provided by water movement may also aid fish digestion and metabolism, and offer other benefits of physical activity. Tropical aquarium fish originating from streams and rivers have evolved to live in fast-moving water and will tend to either grow stunted due to lack of fast-moving water currents or will not grow their full muscular system due to the lack of demand needed. Recreating similar water movement conditions may encourage the display of natural behavior, as well as coloration and physical development not seen in still or slow-moving water.