Discus "Flapping" Gills. Appear to be Breathing Hard

Like the title states my discus appear to be breathing hard, flapping their gills. I have 4 mature discus with two large angels in a 75 gal. Water parameters are the same as always for the past two years. Everybody is eating. Good colors. Just seems like they are laboring to breath. In this tank I have three medium sponge filters going. Temp @ 84-85. low nitrates. I do 25-30% water changes every three days.

Any ideas?

Thanks,

Any other signs of hypoxia?
Hanging out near surface where O2 highest
Swimming less
Eating less

Send video of behavior.

Have you used meds recently? How long have they been doing this and what have you done recently to the tank?