advice on cardinal tetra issues?

might be water chemistry which include ammonia,nitrate etc. PH or water not being soft is not the problem here. i have those fishes in hard water with PH of 7.6

plus tetras don't really need to eat allot. feed them less or once a day.

The betta's reaction is not the main problem, it's how the tetra will treat him. Tetra are known fin nippers and a slow moving betta can be too easy a target. The only one he may be a danger to is the black skirts b/c of their longer fins. But the cardinals, neon's and cory cats shouldn't be in any danger at all. I suggest, should you want to try, getitng a breeder net or box and introduce him them way. Some males are just plain mean, but most are very calm and well behaved with non betta or gourami fish. If you were interested you could look into some female betta. They need to be in groups of 4 or more to establish a heirarchy. You'd probablly have to hunt a bit to get some really pretty ones but they can be quite striking in color even with the shorter fins. They also would be much less of a danger to the tetras and in much less danger from them. Or look into some killi fish, they are surface dwellers and while sometimes hard to find, come in some Absolutely amazing colors! Or, if you decide to forgoe the betta, try some dwarf gourami. They can be very pretty as well. They are a cousin of the betta and can often trigger the male to attack as he would another male betta. Hope this helps!

What is the decor like, and how long have they been in there? If not already done, add plenty of fake or live plant cover, and get your temps to around 82. Give them some time, and let them get comfortable.

ph balance water temp?