So what if you want nine betta fish but you don't want to change nine tanks, and buy nine filters and nine heaters.
Some people really hate split tanks, but I love them.
The dangers of split tanks: fish getting around or over the dividers and hurting each other, diseases traveling through shared water, betta seeing each other and being constantly stressed.
The advantages of split tanks: fewer filters, fewer heaters, fewer water changes, fewer lights. Easier to keep parameters stable for several fish, fewer water tests, less space. A larger water quantity makes waste build up more slowly.
So, first of all, what's a good divided tank?
Here's a hint, it's not this thing.
A divided tank is a tank that has a barrier in it to keep the fish apart. The fish all live in the same tank, and that tank only requires the maintenance of one tank, but can have more than one living space in it.
10 gallon tank, divided to have two living spaces and a small space in the middle.
There are two dangers that you find in divided tanks that you don't find in solo tanks. The first is that if your betta fish somehow gets around or over the divider, there's a good chance that he'll kill his tank-mate. The other concern is that if one fish in a divided tank gets sick, every fish gets exposed to it. Instead of losing one fish, you can lose several. If one heater fails, all your fish will be too cold. You're putting all your eggs in one basket.
Both of these are 100% avoidable, however. Proper quarantining is an important part of any shared tank. Quarantining means you keep the new fish in his own tank for ~30 days, to make sure he doesn't have any disease that could spread. Proper care for a community tank means not putting in any fish you know is sick, or you know was exposed to a disease.
You can keep bettas on their side of their dividers by building the dividers properly. The dividers you see at the pet store don't work for betta fish. Those dividers are built for fish who don't have the biological desire to murder everything they see. The good news is that you can easily build the kind of divider that does work for betta fish, and they're cheaper than the ones at the pet store.
A 20 gallon long tank divided 4-ways
First, you need to think about how big of a tank you want. I recommend having at least 4 gallons of water per fish. This is based on the total water volume. The tank might be divided, but they'll all be breathing the same water, so you need to have enough of that. The other concern is space. Take a very critical look at the size that each apartment will be when it's divided. It needs to be deep enough, wide enough, and tall enough. For this reason I say that a 20-long tank can be divided into four, but a 20-high tank should only be divided into two.
Also you need to take into account your fish. If you have a big guy, he might not be comfortable in 1/4 of a 20-long, but other fish will have plenty of space. If you have a very aggressive fish that will spend hours trying to attack the shadow on the other side of the divider, he might not be happy in a divided tank. Just because a stranger on the internet said it'd be okay for most fish doesn't mean it'll be good for every fish.
I make mine out of plastic needlepoint canvas, the spines to report covers, and a shitload of aquarium silicone. You cut the canvas to the shape you need, put two or three layers together. Then you snap one report spine on the top, and silicone two more spines into the tank. Then you slide the canvas into the spines, and cover the bottom of the divider with gravel or sand. There's tutorials out there that go over this exact process.
Center two apartments of the divided 20. The left and right walls are needlepoint canvas dividers
Using silicone to attach the dividers to the walls of the tank is very important. Betta are crafty little fuckers and they can find their way through the gaps in the wall. It's very important that you also use aquarium silicone. The bottle just saying 100% silicone isn't good enough. Go to the hardware store, find an employee, and ask for aquarium-safe silicone. Then make sure it says "aquarium safe" on the packaging. If they don't have it, buy it online. A lot of silicones that you can find in the hardware store have anitfungal additives that will kill your fish. This is a case where buying the wrong product will kill your fish, so be careful.
If there won't be a lot of jump space when your tank hood is closed, you only need one divider. If there will be a lot of space, you can use two dividers with a little space to catch an angry jumper.
Divided 10 with a thermometer in the jump space
If you want to temporarily un-divide your tank, you can slide the needlepoint canvas out and leave the spines glued in. This will let you re-divide it without having to drain the tank and glue in new spines. If you decide that you want to un-divide it long term, you can remove the silicone cleanly with a razor.
As a final note, I've been told that you should not keep a male and female betta in a divided tank, because being close to each other will keep them constantly ready to breed and stress them out. I have never tested this, but if you are interested in keeping a boy and a girl in the same divided tank, this is something you should research first.
And remember that a healthy fish is prettier than the prettiest tank in the world!