the only thing i came up with personally, and this was sort of conjecture.... was the "irritant" thing. maybe.... bettas are pretty commonly kept in crappy containers with little or no aeration, etc. if you bottle the "fumes" up and force the betta to breathe it, it may irritate their "lung" too much. possibly even causing them to sufficate (sp?) or "choke" on it. this is running on the assumption it's an irritant........ we find the smell "minty", they might find it toxic.
most other labyrinth fish aren't "kept" in small stagnant pools of water. in a larger aquarium, with more movement and possibly aeration this might become a non-issue.
normally the precautionary stuff i see bettas kept in is "green goop". i forget what the stuff is, but it looks like net dip. i haven't used it in awhile, but i don't recall seeing melafix being green or blue.........
most other labyrinth fish aren't "kept" in small stagnant pools of water. in a larger aquarium, with more movement and possibly aeration this might become a non-issue.
normally the precautionary stuff i see bettas kept in is "green goop". i forget what the stuff is, but it looks like net dip. i haven't used it in awhile, but i don't recall seeing melafix being green or blue.........