Have your fish been sick lately? Perhaps a bout of ich that took longer to get rid of than usual? Have you considered it might be your heater?
"Value heaters" or improperly sized heaters may not be able to maintain their set temperature and cause aquarium temperature to fluctuate several degrees per day. A sharp drop in water temperature is extremely stressful to fish and can compromise their immune system. In this weakened state, fish are more susceptible to illnesses and parasitic infections such as ich.
To ensure stable water temperatures, invest in a quality heater, thermometer, and an extra heater as backup in case of emergencies. Choose your heater based on the size of your aquarium, using 3-5 watts per gallon as a general guideline. Maintaining stable water temperatures with a quality heater is one of the simplest ways to create a healthy and stable aquatic environment for your fish.