Anyone know about keeping fish and aquariums? I may need help

I’ve got a betta fish, and I originally had him and an aquatic plant, which I’ve had since September. I haven’t had any problems. A couple of weeks ago, I added a snail and a second plant. I forgot to quarantine them first (but also I live in a dorm and that would’ve been difficult to do). And now I have these dark brown/black worm-looking things in my tank, and I’m not sure what they are. I’ve tried doing research and honestly it’s not been that helpful.
I plan to deep clean the tank tomorrow, and I’ve got all of my plants and animals separated in clean water for now. Just trying to make sure this isn’t too serious.

(Also I’m so sorry if anyone saw all the typos in the original, unedited post. I’m doing this from my phone.)

Yeah, I’ll call the place I got the plant and snail from today and see what they say. I caught some of the worms in a cup for examination too, so that should help. I may do a deep clean anyway because the tank isn’t huge, plus I’ve taken the water out anyway. Thanks for the advice!

I’m lowkey obsessed with medicine too. I’m in public health.
But yeah, I called the store and they said what I already thought was the case, so basically I just had to clean the tank thoroughly. Also probably going to probably wait a bit to put the new plant back in because I’m not sure if there’s a chance that old food and stuff that they like to feed on might’ve trapped in there.

Yeah, apparently it's a semi-normal thing that happens, and it's only bad if you're trying to breed fish because they might snack on eggs. They just sometimes get out of control if you wait too long in between cleanings or overfeed. Which I didn't know because when I used to keep a tank (it's been years, and then I got my current betta in September), I never used live plants. Honestly an easy mistake to make. I'll probably not buy plants from that place again though because they keep them in the tanks with their fish, whereas I've seen other places that package them individually so it's got a more sterile environment.
I've just done my deep clean, and I'm letting the newly-treated water sit until tomorrow. I also apparently have a second snail now, so that's fun. Think it hitchhiked on the plant too lol.