Not enough flow with only one powerhead. You need to get flow all over, including in the lower part of the tank. Keep your light off for the time being except for maybe an hour around feeding. You can use room lights so your fish aren't in the dark. Get a new light bulb and old lights produce poor quality light spectrum which can contribute to the problem. Are you feeding dried foods? If so, stop that and use frozen foods. Dried foods can contribute to the nutrient levels in the tank which is a food source for the cyrano. Instead of doing 3 gal water changes every week, do daily 1 gal change. You need to be sure not to use tap and also be sure to use well mixed salt water. That means mix the salt for min. 24 with a pump. You can also syphon off the cyrano, but unless you improve tank maintenance and water quality in the tank, the problem will continue. Do you use live sand and live rock?
What chemicals are you referring to? Dechlorinator? Can you post your nitrate and phosphate levels?