Betta Fish: Could I have done better?

She had some kind of worms. I didn't have the needed medication till just now. I halfed the dosage since she was in a 5 gal when I was treating her. Before the medication I was treating her with aquarium salts, in leafs for tannins. She had red worm, something I've never dealt with. And I thought I could handle it on my own, I did my research, I looked at different medications before settling on Expel-P. Mind you she had a wound on her cheek that I was treating, so before I realized she had worms. I assumed she was sluggish and not feeling good because of that, but once I added the halfed dosage to the tank, I sat there in waited. I didn't think anything would happen but I then witnessed her suddenly having a seizure. I panicked, thinking I did something wrong, so I immediately did a water change, a small one to lower the dosage, but by then she stopped moving and gill function had stopped. I gently moved my hand under her and tried to hold her, which she never liked resting in my hand. But she didn't move. The pack didn't say it was for salt water fish only, or anything like that. Just that it was ment for treating parasites in things like wasting disease. Could have the medication from the salts or the leaves have caused this? Or was it just a 1 in million chance and she had a allergic reaction to the medication. I had used the other half of the medication in my 10 gal with my male guppies since I've been dealing with occidental wasting disease. And they're still active, like nothing happened just before this happened to her.