I been seeing a lot of thread pop up lately about Neon Tetras housed with Bettas. While I generally discourage this I would like to give a little advice to the newer fish keepers on stocking these fish.
Neon Tetras are not beginner fish. I don't care what the pet store tells you. They are not hardy and are riddled with many genetic defects. These guys will bring home many nasty diseases. Quarintine is not a suggestion with these colorful little fish. It is the rule. You must prepare well once you decided you want to add them to a tank. The tank must be cycled. This is very important. They need a 10 gallon for a school of 6+ and please don't even think about getting less then 6.
You need to pick where you buy these guys well in advance of purchase as well. If they have a high die off rate you want to pick another store. I would recommend Jumbo Neon Tetras or Cardinal Tetras over the standard Neon Tetras. They are healthier over all and have less nasty over inbred genetic time bombs hidden in them. (Yes inbred not line bred.) Neons and Cardinals can be schooled together.
Once your new Neons have spent a month in quarantine it is time to introduce them to their new tank. Divide the tank with a mesh so the Betta can see his new tank mates. Let them spend a week this way so everyone can get use to the idea of sharing a tank. On the day you remove the divider make sure you have a day at home to observe. Watch your Betta closely. He will tell you immediate if he is insecure with his new tank mates. Chasing, hiding, and over aggressiveness are sure signs he is not comfortable with the new arrangement. If he doesn't settle down after a couple hours you have should redivide the tank. You can attempt another week of dividing the tank and repeating the process. Just remember your Betta may never be comfortable with the Neons and if this doesn't work after two weeks I would suggest rehome the Neons in another tank.
Now I will grant there are exceptions to every rule and fish stocking does have a few. Just because you had success in the past with random unplanned stocking please remember our fishy friends are much more susceptible to stress then we are. Please consider their needs. I hope everyone has success in finding the stocking combination that works for you and your aquatic family.