toxic tank now what?

the eel was in a critter tote floating in the tank so he could not have been the culprit... hes still a baby. the tank has been up almost 2 years so i doubt it was a hitch kicker.in addition to that the fish started to become lethargic. ive been testing the tank everyday and still testing zero.... and twice a week at the lfs and they say 0 aswell.as for boiling the rocks ii know these 2 rocks will need to recure but they are only about 2 lbs each and there is about 100lbs. so there should be plenty of healthy bacteria in the remaining rocks to support a damsel.i know i took a big chunk out of the tanks bacteria population but i know i couldn't of wiped out the entire population. i would think damsels should bbe able to live through a cycle anyway,seeing as so many ppl put tthem through initial cycles.

 

well i opted for another huge water change. this time about 75%. that was 3 days ago. added a yellow tail yesterday still alive and doing well, no signs of distress. i tihnk the tank may be ok now. the 2 leather corals in the tank have amazing polyp extension now. more extended then ive ever seen them.