Treatment: Immediate removal to a cycled tank without the toxins.  As an alternative (and only in emergencies), add sufficient dechlorinators to the tank in dosage appropriate for its volume.  Make sure this dechlorinator also removes chloramines as well as chlorine.  Always premix the dechlorinator in some tank water if you are adding it directly to the tank, and ensure wide dispersal.  Increase the aeration to drive off the toxins and relieve Hypoxia.  Check the water for ammonia within a day or so, as the biological filter may have been damaged.

Background: This type of poisoning usually follows the addition of untreated tap water to the aquarium.  No excuse for this one!  TSK TSK!!