Good greif i don't mean to sound blunt but no matter what specise or genus it is it will become enormouse, you were told this before you bought them and you are being told this now, why do you continue to convince yourself you can permanently house this fish?
I honestly believe that not even the seriouse monster keepers on this forum would have the facilities to do so. My advise is to return the fish NOW and let the shop know that they shouldnot be selling these fish at all let alone to unknowing customers.
Sorry for the rant but to me you a being very irrisponsible, please re-think your decision.