[Amphibious]

You and your Hippo Tang may be sorry if you introduce an Achilles Tang into a 90. The Tang family and the Achilles in general are very territorial and there will be severe challenges between the two Tangs initiated by the Achilles. It could lead to the death of one. Secondly, as Jeremy pointed out, your 90 is on the small side for even one Tang. Tang's are wide roamers in the Ocean, traveling in large aggregations on the look out for food. Keeping one in any size home aquarium is a serious reduction in their usual range. That brings out the territorial fighting. You must also take into account that your Hippo Tang reaches a length of 12" and the Achilles 15" in the wild. I would not recommend the Achilles even though the price is right.

Dick