I'm not a betta expert, but from what I've read here and at other expert sites the last couple of days, I think you should be changing the water much more frequently. I mean, I have a single betta in a filtered, planted 10 gallon tank, and I was told to change 50% once a week for regular maintenance! o.o
Also, some sources are now advising that melaleuca/tea tree oil/bettafix/melafix may be dangerous to anabantoids. Some advise to use garlic juice
as a natural preventative/treatment instead. I don't know if this is really true about the melaleuca, but it's enough to make me not use it anymore. If this is not accurate, I'm sure I'll be corrected..
And just from general fish knowledge, higher temps will rot uneaten food and waste faster. Higher temps will also make free ammonia in the water more poisonous to the fish, as will a higher pH to a much greater degree. What is the temperature and pH of your water? (If the pH reading maxes out with a normal kit, get a high-range pH test kit. I've always preferred Aquarium Pharmaceuticals liquid test kits.) Does it contain chloramine or only chlorine? What is the ammonia reading before and after water changes? There are ammonia-toxicity charts floating around the web that will let you know just how dangerous your water is at any given time, knowing these readings:
To lower pH, if it is high, you can partially use distilled water, or you can use blackwater extract, which I'd recommend preferably, as it contains tanins. I haven't been using it for my betta, but I'm going to start now that he's been sick a couple times (my pH is very high.)
Unless there's something I'm not aware of, why would a real, non-toxic plant be harmful? Ammonium is the first food of choice for aquatic plants (don't know about terrestrial plants.) I know for sure you could obtain a large Anubias barteri that was grown emersed
on eBay, and it would live just fine with only the rhizome and roots in the water, if terrestrial plants are indeed some sort of danger. You would have to add a drop of something like Seachem Flourish for the plant (IMO, it's the best plant micronutrient additive out there.) It will also appreciate blackwater extract.
[addendum: I just use Kosher salt if needed, but not usually, as it's not good for plants.]
I hope that helps!