Most pet shops sell bettas in tiny jars. However, like most fish, bettas need a real aquarium to thrive. Since male bettas only attack other male bettas, they can even live with other fish in a community-style aquarium. While filters improve water quality, which promotes fish health, bettas are generally not strong swimmers, and need filters that don't generate a lot of water movement. Several varieties of filters exist which fit this bill.
Canister filters offer several advantages for an aquarium with a betta. Many models have controls which allow you to control their return flow. This is important in a betta tank, since they cannot swim against strong currents. Since many models are available, make sure you pick one that has adjustable return flows. The biggest drawback with this kind of filter is the cost; they are very pricey compared to other types of aquarium filters.
Hang-on, or power-box, filters may or may not work for betta tanks. Many models have strong return flows, which can interfere with bettas' swimming. However, some models have biowheels, which are designed to give beneficial bacteria a place to grow. As a fringe benefit, the wheel also dissipates the return flow, preventing strong currents. If you get a hang-on filter, make sure you get one that has this feature if you plan on using this filter for an aquarium with a betta.
Undergravel filters can work for a betta tank. These filters are powered either by an air pump or an underwater pump called a powerhead. If you want to use this type of filter for a betta, use an air pump. With a cheap valve, you can adjust the air flow, and produce just enough water movement to run the filter without creating the strong currents that cause so much trouble for bettas. Do not use a powerhead, as they create too much water movement for a betta.
Sponge filters are an old-school filter that's been largely replaced by better-looking filters. Sponge filters take up space in the aquarium. But while they're ugly, they are useful for utilitarian aquariums like breeding tanks. If you are breeding bettas, these filters are very useful in that they have no dangerous inlets which can suck up baby fish. Like undergravel filters, they are powered by air pumps, and you can adjust the flow to avoid excessive water movement.
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