You mentioned Melafix--have there been any actual studies on the effects of tea tree oil on the labyrinth organ? In the past, I've used
Melafix with anabantoids with no problem. Part of me has always suspected that maybe a few hobbyists had bad experiences with Melafix+anabantoids, speculated as to what caused it and then passed on their conclusions as facts. It's not something I've spent a lot of time reading about though.
I have been looking for actual studies for a long time, however, for many reasons its just not a common study topic. The effect of the oil on mice and rats is pretty well studied because of its direct connection to human use. However, with fish the main reason something like this would be studied is for its use on farmed fish. Bettas and other labyrinth fish are not farmed on a large enough scale to generate funding to study something like this, so it is largely unstudied.
On another note I did find a pretty good blog with someone who has put together a decent study of anecdotal evidence mixed with scientific understanding and some of their own observations and small scale experiments and this is what they had to say:
My current research and hypothesis, is that since the best research shows similarities between TTO and Turpentine (both are terpenes), is that in an acidic environment, in particular an environment with nitric acid or other acids as a result of organic decomposition such as carbonic acid, the chemical reaction can produce chemicals that may harm the liver in certain fish that have a tendency to ingest the water around them such as Labyrinth fish/Pencil fish (via the surface).
Certain terpenes such as turpentine are actually explosive when combined with nitric acid (this chemical reaction is used in rocket fuels!). On a VERY small scale (aquarium environment) some similar reaction may be happening that with certain fish can cause death.
This would also explain why this problem has never been noted in marine fish even though they constantly drink the water around them, since marine fish are always kept in an alkaline environment.
http://www.fish-as-pets.com/2008/12/melafix-dangers-labyrinth-fish.html
Basically, if you keep your water neutral or slightly alkaline, you shouldn't have much of a problem. But in an acidic environment you
may have a problem occur. There are other various factors that are mentioned in this article as well. Its a good read if you have the time. They also mention that it is possible the medication itself didn't work, resulting in a death from the infection and the user may have attributed the death to the Melafix. This happens a lot. . . However, I still prefer to err on the side of caution and do half a dose of Melafix/Pimafix with bettas and scaleless fish. If the infection is mild enough that the oils in these meds will kill it, then it will do so at half a dose as well. Especially if you're just using it for wound treatment a half dose is fine with most fish.