Thanks for the responses.
I have an overflow and sump with 6 or 7 times tank volume falling through it every hour. Total water volume about 57 net gallons. I did just increase this flow and it has resulted in more degassing at night. It used to degass to about 7.3, now to about 7.6. Fish load has decreased due to death to one 4" bristle nose pleco, two 2" siames algae eaters, and five tetras. I've had at least ten fish die one by one over the last few months.
I have measured dissolved oxygen and it's sufficient; 8 mg/l on a Salifert test kit. With the increased flow I assume it's a bit higher now.
I am thinking the problem was my previous 1.4 drop from 7.8 to 6.4? And now 1.3 is still too much for the fish?