I found that with the pair of hippo I had before that keeping the temperature ROCK solid was the best way to keep them healthy.
The garlic method has worked well for me though. I have used it for them, and for other tangs I have added to other tanks with great success so far.
I am lazy, so I usually use Lowry's Garlic Extract. I just add it to some food, and let it soak in the fridge for a day or so. I found by letting it soak in that it reduces the amount of oil scum you can get on the water surface.
The way I do it reduces the effectiveness of it in treating ich. But it has always worked great for me, and causes little to no negative effects.
Note, I have tried feeding it in MANY different ways at first until I realized the temp changes at night played a bigger role.
The BEST method is to physically crush the garlic right before feeding. It is the actual desctruction of the cells within the garlic that releases the Allicin. And also note that in this form Allicin only has a certain effective time limit. I do not know what hte limit is though. And even the 'old' garlic extract works for me.

Just my 2¢
Ed