Sound advice from Maria Nepen and I agree totally with what she is saying another thing you could do here is sniff the water , then tell us what it smells of, if the answer is nothing its an indicator that your water is fine but back that up with a drop test check on all your perameters .First thing I check when fish are acting off is the water quality (using test kit) and if it isn't a water quality issue then check for parasites.
By any chance have you recently added a new fish to your pond?
If it smells then its an indicator that your water quality isnt good again back this up with drop kit tests .
It's a little used thing in fish keeping nowadays almost a forgotten aid to good water "I use it all the time".. George Orme Author of the Book Goldfish and Koi ISBN 0862300207 touches upon it .
If all is well water wise then its either down to water temperature or Preditor's especially if they are not unwell .
Dave