Just came from the Petco. It sad out of 20 bettas left may be 2 are healthy , others look very sick. If you buy two bettas please quarantine each. Do not share anything between them and your other bettas in case it sick. Sometimes symptoms can show later. And actually i would not even shae anything between two new fish.
And even when you buy live plant sometimes , yes rarely, but can happen that plant can carry ich. So if you buy live plant be careful, and try at least quarantine it for 2 wk in the water without water conditioner. Check post #10
I also want to give you one link that i like you will find helpful information including best recommended food.Also not sure if anyone mentioned in that thread about frozen food. It enriched in protein and very good. Just need to be careful do not overfeed. Frozen blood worms and frozen daphnia are good.
When you do full water change make sure you acclimate your betta. You will find acclimation instruction in that thread .
As for the small tanks. Almost all my tanks are 2.5 gall . I have only one 5 gall tank. I have 10 bettas. Well 3 of them at work though. All my tanks uncycled. I don't have and never had filters . Well only one time when i had 10 gall long time ago.And my tanks at work are 1 gall tanks. I have one fish that is 5 years old in 2.5 gall tank, some of them 2 years old and i have 3 years old, etc... So it really matter of keep the water changes up . And i tell you that no matter how much you do right care the bettas still will have different life expectancy depending on many factors including the immune system . I had 2 bettas i bought at the same time and do the same care , one is 3 years old now and another one died long time ago. So...
2.5 gall is good size for the betta. Many people saying that it possible to cycle but it easy to crash the cycle in the small tanks. So i am the one who will not recommend to cycle 2.5 gall apparently my bettas live happy , healthy lives without filter. Make sure you acclimate them though. For 2.5 gall you can do 2-50% and 1-100% water change. Or you can do just full water changes every 5 days.
I never recommend very warm temp (warmer than 78*). I am going to copy some ones post that i am agree with
''The best IME is 76F-78F. Having the water warmer will make their lifespan shorter because they're growing so fast with all the heat. I say 80F as maximum if you want your Betta to live a little bit longer.''
And also the warmer is the temperature -more chance for the infections.
And for your 5 gall tank make sure you do 50% water changes weekly and you always can do more if you need to. Make sure you vacuum the gravel and swish/rinse filter in the tank water weekly. Filters can carry all kind of gunk ...So make sure you rinse it and if the cartridge becomes too old or starts to fall apart, put a new cartridge in the filter but keep the old one behind it for a few days and monitor for amonia spike.