Here is some information i have stumbled across in my own reading up into Melafix, especially useful if you are making your own brew
This was stated by a qualified aromotherapist on another site i found (whether he is qualified etc, i don't know).
The essential oil in melafix is cajuput NOT Tea tree. Botanically cajuput is Melaleuca cajuputi, also known as Melaleuca leucadendron. Tea tree is Melaleuca alternifolia
Contrary to popular opinion it is not made from the common Tea tree Melaleuca alternifolia but from Cajuput which is Melaleuca cajeputi or Melaleuca leucadendra. This is confirmed by the manufacturer's Safety Data Sheet taken quote from wikipedia
Whats the difference? Chemically cajuput is about 40% 1,8 cineole. Tea tree is about 45% terpinen-4-ol. Cajuput is more closely related chemically to eucalyptus! Eucalyptus globulus, probably the most common eucalypt used in aromatherapy is about 65% 1,8 cineole.
Both cajuput and tea tree are antifungal, antiviral, antibacterial and more. I have used tea tree in my tank to good effect. Cajuput is chemically gentler and probably the better of the two. There have been cases of humans being poisoned by drinking tea tree, lol cant understand why you would want to drink it.
There are no claims to my knowledge that tea tree or cajuput will cure ick. Having said that I treat ick with salt and temperature as has been recommended here. Huge thanks for that info

I add 1% solution of cajuput at 5ml per 40 litres as recommended by melafix. My opinion is that helps heal the wounds caused by ich and helps restore the fishes slime coat. It may kill ick as well I really dont know. It certainly helps the fish.
As has been stated here as well, buy your essential oils from a reputable dealer. Be very careful of supermarket versions, they are often diluted and contaminated. Thursday Island tea tree is ok, although a little rough. Far better to purchase pure essential oils.