The male Siamese fighting fish (aka Betta) was made by hybridising Betta splendens with Betta imbellis. Betta splendens is very aggressive towards males of their own kind, whereas Betta imbellis is peaceful and can be kept in groups. Because of this, some male B. splendens are actually quite peaceful (due to having the peaceful traits from B. imbellis). However, if you put a mirror on the side of the tank, your male should flare his fins out and try to attack and drive away the intruder (his reflection). So even tho he might seem peaceful, if another male comes into his territory, he would probably try to chase it away or kill it.
You can actually put a mirror against the outside of his tank and leave it there for about 10-15minutes, then remove the mirror. Do this once or twice a week. Your male will see his reflection and consider it an intruder and show off in an attempt to drive it away. After 5-15minutes you remove the mirror and your male thinks he has driven the other male away. He will then strut around the tank for a bit thinking he is a big man, before settling back down to his mellow ways.
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In a reasonable size tank with lots of plants, you can keep some fish with male Bettas. However, you need a tank that holds at least 40 litres and you need to avoid fish that are active, nip fins, or are related to Bettas.
Fish to avoid include: danios; barbs; serpae, black widow, Beunos aires & blind cave tetras; gouramis and rainbowfish. The danios, barbs & rainbows are too active for a Betta. The tetras are fin nippers and some barbs are too (mainly tiger barbs). The gouramis will be treated the same as an intruding male Betta and get attacked.
Shrimp and snails are normally fine with Bettas.
Endlers livebearer and other livebearers like guppies, mollies, swordtails & platies require hard alkaline water. Whereas Bettas come from soft acid water. So avoid Endlers and other livebearers.
If you wanted to try some fish, and the tank is at least 2 foot long and has lots of plants, you could look at a group of tetras (neons, black phantom, glowlight, black neon tetras) or some small catfish (Corydoras). If you visit your local pet shop they might have male Bettas in some of their tanks with other fish. If they do you should be able to keep those types of fish with your Betta. Having said all this, you still have to watch your Betta during the first few weeks after adding any new fish. Some are fine and get along great with other fish, and some turn nasty.