I disagree about hippos being harder to keep. Unless I am just very lucky. I have a tiny tiny hippo(nickle size when bought now about an inch and a half), a purple and a sailfin. All these have had a small battle with ick when first introduced but I haven't lost any. The purple was a little longer battle but I believe that Achilles would have to be higher than the hippo. I also have come to realization though that if my tangs are eating, I don't remove them from the tank. I think this only stresses them more etc. Maybe I have just been lucky with my tangs but I would say my hippo is pretty ich resistant. Anway I would say my purple is a little more suspetible to ich than my hippo.
Ps I also qt for about a week, which I know isn't long as it should be but my qt is only 10gallon and I feel like I am cramping their style. LOL gonna try a goldrim or Achilles next when I get my large tank. Prolly goldrim at this point though. Wow sorry I got real off topic there.