If the tank has not completed its cycle yet it will be a challenge to keep NH3
at levels that will not kill your fish. Frequent PWC
will help but they will prolong your cycle drastically. Best practice is to take the fish back to the LFS
and let your tank cycle natualy and completely before slowing adding any livestock. It would be better to get some store credit for the fish as oppossed to letting them perrish in your tank. As mike stated, a 46gal is simple too small to keep any tang. It is not too late to get started out on the right foot.