I suspected that you had ceased feeding. That is why I inquired about the fish being at the pond surface. Obviously, if all of the sessile algae is gone,your fish are still looking to feed. If they have ingested any parts of the Oak leaves that fell into the pond in any meaningful level, It is possible that they have become constipated. This is typically the cause of swim bladder issues, but this is rarely fatal.
I am more concerned with the low GH. GH and KH levels should be about the same and at least 70 ppm. Your current KH of 120 is very good, but the GH needs to be raised to an equivalent level. This can be done through the addition of Epsom Salts and Calcium Chloride in equal parts. The typical dosage is One (1) teaspoon Epsom Salts and One (1) teaspoon Calcium Chloride per each Five (5) gallons of water. I would recommend applying this dosage daily until the GH has reached the desired level. Do not exceed this daily dosage as, is the case with all other water quality parameters, change should be slow to minimize stress/shock.
Epsom Salts are available universally and Calcium Chloride is available from any Swimming Pool supply, even from Walmart.