What's in the tank now? I wouldn't add any medication if there is sand, live rock or any inverts in the tank. Maracyn is an antibiotic, do you see signs of bacterial infection? Cloudy eyes are often a sign of poor water quality, which is certainly a possiblity considering you just lost a bunch of fish (anything dying in the tank could cause an ammonia spike). My advice given the information provided is to test the water for NH3, NO2 and NO3, do water changes if NH3/NO2 are more than 0 to get them down, and do anything you can to reduce stress on the cardinal. Feed him high quality foods, try to limit activity near the tank, turn lights on/off gradually so he's not shocked, keep the water quality as close to perfect as you can and cross your fingers.
If the cardinal dies, I'd run the tank without fish for six weeks to deprive any parasites of a host and cause them to die off (it may have been the temp spike that killed the fish, but it may have also been something like ich, brook or another parasitic infection triggered by the stress of the spike), as well as give things a chance to stabilize if the water quality is less than optimal.