I highly recommend you put this together in a dry, barren tank. It would be no fun trying to create this with water abstracting your view.
There are several ways to make this tank divider. One way is to make a one-sheet divider like the ones they sell in stores. I think the more space you put between the fish, the better. So I will go over what I did for my tank, making a double sheet betta divider. For a 20 gallon long, you’ll only need two sheets. For a 10 gallon, you might still need two just in case. For a 20 gallon tall, you will need to buy at least one more sheet, two to be safe.
1) Measure and Cut
So you got all of your materials. Time to start measuring and cutting your mesh. Because the poster boarders are going to clamp the sides of each ( to make it sturdy), keep in mind that there will be an extra centimeter or two for the plastic boarders. Constantly check by adding the poster boarders after every cut and place it in your tank. I accidentally made mine too short, but I will go over how to correct this later.
2) Secure the Mesh with Suction Cups
So you got your mesh sheet with clamped poster boarders. You’ll want about an inch between the sheets, so it helps to measure and mark the top and bottom part of your aquarium (could mark it over tape ) so you know exactly where you want your sheets to be. In my example, I used eight suction cups with four on each side. I lined up the cups, making sure each sheet has a top and bottom cup attached with fishing line. This is the difficult part, because if you make the line too short, the poster boarder will be over the suction cup, creating a gap and possibly a warped sheet.
3) Fix Any Concerns Once It’s Created
So your finished product looks a lot like mine. The suction cups are pressing against the sides, creating gaps between the cups, or you simply cut it half an inch too short. If you think there is a possibility that your fish could get through, then they can. But don’t worry; I have a few solutions. For one, if the gap isn’t too large, measure and cut up a piece of plastic straw and simply squeeze it between like I have done on the right. If the gaps are too wide and the pieces of straws loosen and float up, use any remaining mesh and simply tie it outside of the sheet (like I have also done). Don’t tie it on the inside, because the fish will be able to press through it. By tying it outside, there's no way they can push through. Make sure it's tied as close to the original mesh as possible; you don't want it to be loose. Adding gravel and leaning heavy décor against it will also keep it secure. For the sake of lowering visabilty of the other fish, add some unused aquarium plants between the mesh.
That’s it. Just make sure there is no way for one of your fish to get through to the other side. Once you have built it and made necessary alterations, keep an eye on the tank for a while. One of your fish might find a way, and you want to catch the mistake sooner than later. Also, when doing water changes, you’ll want the water level to at least be 1.5 inches lower than the top of the mesh. Remember, bettas can jump. Enjoy!