It works best if you combine it at a rate of 1ml tea tree oil to 100ml of R/O, distilled or cooled boiled water in a bottle and shake it vigourously for a couple of minutes before adding it to the tank. If you add the pure oil direct it will float on the surface and do very little.
I thought oil was lighter than water so they needed to add some alcohol when making melafix so that they could mix... (alcohol mixes with water and oil)
Thats right, to permamently bind oil and water you need an emulsifier of some kind to stop them from seperating again which is why melafix add a tiny ammount of alcohol to the product to give it a shelf life.
With home made tea tree medications you only make up the ammount you need each time so a couple of minutes vigorous shaking is enough to temorarily combine the oil and water long enough to add it to your tank.