Please don't recommend things for a cough that you clearly know nothing about.
Tonic water absolutely has a medical history. It's not hard at all to find that out by reading the label and doing a little online research.
It makes me extremely uncomfortable for you to talk about tonic water for a very long list of reasons.
Because it contains an actual for serious drug -- quinine -- you can get serious side effects from overusing it. These include vision problems, headache and nausea similar to migraines. IIRC, you can cause yourself permanent vision problems.
Because it's a powerful alkaloid, overuse can cause your stomach to become too alkaline. This can cause you to be unable to digest food and can result in vomiting.
The name "tonic water" is medical in origin. The bitter flavor you speak of was common to a class of medicinal herbs typically referred to as "bitters."
Quinine is a drug related to the drug currently being tested as a possible treatment for covid19: Chloroquine. So casual and uniformed use of it could create resistance in the virus, making actual medical treatment less effective in the future.
I highly recommend you keep a journal and actually read up on anything you are finding personally helpful.
I'm not here trying to tell people what to do with home remedies for casual use. My consistent framing has been "If the shit really hits the fan and doctors have nothing for you, there are other options that may make sense to gamble on if your options are take a chance or die."
I know a helluva lot about medical stuff. My mother wanted to be a doctor and she personally changed the practices of two cancer clinics when she kept my dad alive after they wrote him off for dead. I have another relative who works at the CDC and has for years.
I've been surrounded by medically knowledgeable people my entire life. I also absolutely don't hawk home remedies.
I've spent a lot of years trying to figure out if there is any way to effectively share what I know about CF and other health issues to either be helpful or to somehow get actual medical professionals and scientists to do studies.
I would love to finally have credibility and be taken seriously in the world and have actual respect.
And it matters vastly less to me than avoiding potential harm to potentially millions of people because I'm cute and charismatic and people want to be my fwend and it goes amazingly bizarre places and always has.
I know you mean well. I know you think I'm pathetic and sad and a social outcast and a poor person and you think you are doing something nice by publicly patting me on the head.
I absolutely don't see it that way. I'm extremely uncomfortable with you and other people here clearly desiring some kind of feel good emotional attachment to me personally as your primary goal of engaging me, very much at the expense of best practices for disseminating medically useful information.