i made that mistake as well. i had two maroon clowns before the tank was cycled, and one died that night, the other a few days later during an ammonia spike. my tank took about 2 1/2 weeks to cycle properly. i would say take the fish back to the lfs
. if you have become attached to the fish, then just ask them to hold them till your tank is done, so as to keep the beautiful fish alive.