Water Slightly acidic
Plants/caves to hide in, rest on.
I have mine in a bowl with aquasoil, some plants and Marina C10 heater (real small, has built-in thermostat) and a small in-tank filter. He's been doing good for a year now. Never been sick/fin rot/lethargic. I use the same RO water I use for my CRS shrimp.
Just be sure to baffle your filter output, male bettas stress when they have to fight the current due to their large fins. They also do not like surface agitation. My male betta loves plants floating at the top to rest on and hide in. I need to pick up some floaters, but for now I'm still floating my anubias that had to recover from BBA and a sword runner that is floating near the surface which he loves.
Also, absolutely do not overfeed them. My betta got popeye really quick when my wife would feed him because he looked hungry. They always look hungry, and are con-artists when it comes to getting what they want by not eating a certain type of food. I try to alternate between freeze dried blood worms and betta bites. They typically eat 3-4 kibbles a day for an adult, it's best to randomly drop one in from time to time to simulate wild feeding. Also pick a day of the week to fast, not feeding for a 24hr period of time. This is good for their digestive system.
I have also read they like the tinted water provided by the Indian Almond Leaves, I may be trying this out soon. I have kept my betta in a 10G for about a year with four corydoras and an otocinclus without any problems. He's always swimming around checking out what is going on in his tank and loves being the center of attention. I guess I got a betta that doesn't like making bubble nests though, which is a bummer but he is very happy and healthy.
i dont reccomend freeze-dried foods for bettas, except as occasional treats (i will give freeze-dried bloodworms to adults once every other week). they can cause digestive issues like serious bloat.
tannins (what the leaves release) can be useful as they soften the water, and have antibacterial properties (but i doubt they are significant in an aquarium). but they are not necessary.
also, you dont need indian almond, oak leaves work as well. i used 2 oak leaves since they are free and easy to get.
I keep mine in a 5 gal tank (he's getting upgraded to a 10 gal halfmoon this weekend though) with the heater at 78 and is fed 2-3 pellets twice a day (ocassional frozen bloodworms as treats) and two small pieces of thawed peas on Sunday. 50% water change weekly. He seems happy, blows bubble nests like crazy so I guess I'm doing something right. My water is pretty soft too (PH 7.2, GH 3 KH2).