Assuming you use Prime or something similar? Possibly your water has changed due to seasonal considerations. Keep the lights off, add aquarium salt at 1 tablespoon per 10 gallons until someone comes on and tells you to increase it to 1 to 5 gallons. Add additional Prime to the tank, increase aeration and decrease the size of the water changes you are doing (it is somehow related to your tap water) to less than 50% unless/until otherwise directed. Replace salt removed with each water change (based on the volume of water changed, not your tank size) I had this happen to me about 15 years ago and it's heartbreaking to watch them go through this. Most of mine survived but I had a better med person helping me. Hopefully someone will come along soon. Good luck!