There are numerous ways to make divided tanks for betta fish, and if you want to keep more than one betta in the same tank, then a tan divider isn’t just a nice idea, it is a necessity.
Most glaziers will cut glass to size for you. You can then silicone the glass inside your tank. This is a good, permanent solution. If you aren’t going to filter each part of the tank separately, make sure that you leave a small gap at the bottom of the glass. It should be a large enough gap that your fish cannot swim under it, but enough to allow the water to flow through it. If you are planning to put substrate like gravel or sand in the bottom of the tank, then you will have to either filter each part separately, or come up with another filtration method that works. Cutting up an undergravel filter and fitting it to each part with separate uptakes may work, or using several small sponge filters attached to an air pump with multiple outlets may also work.
Perspex is much the same as glass, but it can also be drilled. (I would recommend that you get an expert to do this as it cracks quite easily.) You can then silicone it in the tank for a nice, permanent solution with good water flow.
DIY Temporary Tank Dividers
If you want a less permanent, expensive, or complicated solution, you may be better off using some of the following materials.
Coroplast can also be used as a tank divider, and is easily perforated to allow water flow.
Plastic fly screen is another option that can be used and makes for a fairly sturdy tank divider.
Plastic perforated canvas also makes for a handy tank divider, and allows very good water flow.
With less permanent or or less solid tank dividers, you can use your imagination. Some people use report binders siliconed into the tank to hold plastic fly screen or canvas in place. Another option is a dollop of silicone holding the divider in place at the top of the tank and having the bottom of it held in place by the substrate.
Foam is also an option, if you can find a thick, inert and sturdy foam, though the thickness of foam may mean that you lose a lot of space to it in your tanks if they are not very large.
Pre Made Tank Dividers
It is also possible to purchase professionally made tank dividers. They tend to be more expensive than the options listed above and may not be available in some countries. Oddly enough, in New Zealand and Australia, it can be quite difficult to purchase tank dividers even for common tank sizes. Lord knows why as there is a definite market for them and they make life a great deal easier.
Still, as you can tell, there are a wide range of ways to make a divided tank depending on your personal tastes, preferences and budget. I would personally recommend either glass dividers with small foam sponge filters in each division, or the fly screen option, which allows you to run just one filter for the entire tank.
Tip. For keep good quality betta fish recommend separate tank for betta fish will better because if some fish got sick.