Well going off your other threads regarding this pond you have, I'd say it's sudden inactivity and loss of appetite are maybe down to water temperature? You seem to be having problems with the (night time) winter temps in Saudi?
You've already lost one RTC and other fish are uncharacteristically lethargic too. "Thanking god" that winter's nearly over is not really the best way of approaching this issue! You need to deal with the now!
Could you give us an idea of the typical differences in day and night time water temps of your pond at this time of year? I don't think you've posted any actual numbers in any of your previous related threads. Those day and night fluctuations will speak volumes.