Question
Hi Jonathan, I have an outdoor water feature/waterfall.  Top section is about 4' deep and 4'x5' rectangular.  Bottom section is 1.5' deep with same rectangular dimensions.  I have 2 koi and 6 goldfish in the top sectionl.  1 smaller goldfish in the bottom section.  Aeration is provided by the waterfall, which runs 5 hours/day.  Water temp in the top is 76 degrees F.  Both sections contain plants, all of which have been in place > 1 year.  
Until about 3 weeks ago, the fish in the top section were ravenous eaters.  They came to the surface when I gave them food (I use Pond Sticks) and attacked everything with gusto.  Now, the fish in the top part ignore the food.  They eat some of it after it sits awhile.  The fish in the bottom part eats more avidly.  All the fish seem healthy, and the fish in the top part chew algae from the back wall of the tank constantly.  Any idea why they're off their food?

Answer
I have a couple of theories.

Is the only aeration provided last for just 5 hours per day?  If so, this is bad both for the fish and the beneficial bacteria that keep your chemistry in balance.  The water chemistry can also directly affect a fish's appetite.

It's also possible the pond sticks aren't providing the proper nutrients for them.  Are you using a summer staple right now?  Also, did you switch to a wheatgerm based food during the colder months and ultimately stop feeding for the coldest part?

It boils down to three possibilities: 1) improper chemistry  2)damaged digestive organs  3)  food to low in needed nutrients.

Let me know if any of my guesses are incorrect, as that will alter my answer.  Thank you.