That's the problem. Cannabis is a demanding plant (for nutrients) and a wee little Betta splendens isn't going to produce enough waste for the plant to do well on.
You're going to have to scale your dreams up my friend, WAY up. 10gal tank minimum, and I'd be doing goldfish or koi specifically because they are such 'dirty' fish (produce a lot of waste through activity and because they shed a lot of slime), and even then you're likely going to have to supplement, which is tricky because there are fish present, it's got to be 100% organic AND fish-safe. If you started out at that scale you could do a plant, maybe two, if you keep them small.
Or, you could scale it up even more and go for a pond with bluegill, like I did. :D I've got me some breeders now!
At this rate I think I'm going to be writing about fish again.