Your betta doesn't like most other fish. Put a mirror in front of her and she doesn't even like herself. Keeping the angry swimmer in an aquarium with another fish -- especially another betta -- is asking for disaster unless you divide the tank into two sections. A divider ideally keeps your betta on her side by preventing her from jumping over or squeezing through.
Creating a Divider
You can create an effective and cost-efficient homemade divider with a plastic mesh screen, typically sold as plastic canvas, and plastic report holders, strips that slide along the left-hand side of a report. Cut the mesh screen equal to the dimension of the side of the tank. Cut four plastic strips, two to match the height and two to match the width of the tank, and then slide them along the sides, bottom and top of the mesh. If the screen tends to slip out of the strips, apply aquarium-safe silicone or super glue inside the strips to hold the mesh in place. If your tank is taller than the strips, cut additional side strips, slide them on the mest so they're seated on top of the ones already in place, and secure them with silicone or super glue. Alternatively, you can purchase a divider from a pet store. If you make your own, opt for plastic mesh only. Do not use fabric, steel or wire mesh.
To install a divider, start off by filling another tank with some of your betta's water, enough so she can swim around. If you don't have another tank, anything that hasn't come into contact with chemicals -- such as a clean bucket -- will work fine. Scoop her up in a net and move her to her temporary residence, then drain her primary tank. If you made your own divider, place it in middle of the tank so the bottom is flush with the tank bottom. You'll need to push the substrate aside to do this. Replace the substrate along the bottom of the divider to better hold it in place. Refill your betta's tank. If you bought a divider, most come with brackets that clip onto the top of your tank. Slide the divider into the brackets to secure it.
Dividing your betta's tank can result in two potential problems. Overly aggressive bettas may sit in front of the divider all day, look at your other fish and flare her fins and gills in attempt to appear larger. A male will also flare his beard. Constant flaring can stress out your betta. Doubling the divider with another layer of mesh or choosing a darker mesh can sometimes alleviate the problem. Some bettas may attempt to jump the divider. You can prevent most successful jumps by keeping the water level about 2 inches below the top of the divider. If your betta turns into an Olympic high jumper or continues to flare up in front of the divider, a divided tank may not work.
Avoid dividing a tank of less than 5 gallons' capacity. Despite the fact that pet stores often house them in cups, bettas need and enjoy plenty of room. The University of Miami Department of Biology website says an area of less than 2.5 gallons isn't enough. If you plan on adding something other than a betta on the other side of the divider, make sure that portion of the tank meets the specific fish's space needs.