With any tang you are going to need alot more live rock then just 18lbs in a 90g tank. If you already have multiple tangs in your tank, keep in mind they need alot more live rock, not just to feed on but as a filtration factor. IMO you can never have too much live rock. With the achilles, many will agree that they are NOT the easiest tang to care for, I believe that they require alot more live rock they what you have. It may do OK in the beginning but long term, IMO, it is not a good idea. In order to keep all of those tangs alive for long term and for them to florish, I would feed them a varied diet including alot of different varieties of meat and veggies. I would also offer them algae sheets every day. If you are thinking about mandrins, I would advise you not to get them until you have enough live rock to keep them. Most mandrins only eat pods of your very mature live rock. You will get the rare individual that eats frozen. I would worry about your tangs being happy and very healthy before try a somewhat difficult fish like a mandrin.
Melly
p.s. garlic does not "cure" ich or any disease, it is used for finicky eaters, and to help boost the fish's immune system. If he was strong enough to fight ich off, they GREAT!! but even if you use garlic and the fish's immune system is damaged, it might not have the same effect