I have tried the achilles tang on two occasions many years ago. In both cases the tangs died.
I read subsequently that the are a very very difficult tang to keep as others have posted. I think it is on the way out as the symptoms you described were the same as mine.
Although they are algae eaters I would add to there diet varied high quality foods including garlic and selcon supplements.
I do know that the require highly oxygenated water so I would suggest you install a wave system or at the minimum place a hefty powerhead six inches below the water line. You will see that the achillies will 'play' in the stream.
Best of success. I hope everything works out.
http://wetwebmedia.com/badacanthurusaq.htm has excellent recommendations on the good and bad tangs to keep. The achlilles tang is on the bad list.
Apart from the well know tank size requirements (which you easily meet), reefers are well advised to consult this list before considering a tang purchase.