Betta Fish: What’s wrong with my fish?

My GOURGEOUS betta is having some issues. It’s my 5 year old daughters, really. 😢 I really was amazed at how beautiful it was when we saw it at the store, I was surprised. Now all of a sudden its fins are all torn up.

Anyways,
It started after I unsuccessfully tried to introduce a few neon tetras. My fish’s top fin looked chewed up, like shredded. And so I blamed the tetras, rehomed them. Oh, and 2 snails but they died pretty quick 🤔
Betta was acting lethargic after the fact, lying on the ground a lot. It used to love its leaf bed way at the top. And would swim around. It just seemed listless. So I tried a few things to help it out. Cleaned the tank, obviously, replaced filtering (ohh but not all of it, just 1/3 of the layers…the carbon. 🤔 I swear I saw small white spots too. Ich? I wasn’t sure, but I treated for it just in case. A handful days later fish is perky, swimming around, socializing. Looks heathy.

But now! Tonight! I go to say hi, and its fins are ruined! What the heck is going on? I’ve been putting Indian almond leaves in for a while now. I’ve been keeping an eye on the water quality. And it’s been very decently good. Like meeting good parameters. The nitrates/nitrites were showing a teensy bit on the test. The past few days. I’ve been topping the water up. The fish/pet store lady said to put a little water primer in it. Like Seachem Stability. So I’ve done that a time or 2. I have some Bettafix, would that be beneficial? Tea tree extract or something? I really hope someone has some advice! ❤️ Maybe I should change the water tomorrow? Like 50%?

I’m devastated. We’ve had it for a few/couple months. And I’ve been trying so hard to be a good mom and figure out how to take care of a fish properly, because my daughter was so excited. 😩