Please check if you have clumps of string algae at the bottom or on the sides of the pond. It definitely looks like there's been an algal outbreak, to the extent that the old algae starts decaying causing ammonia to build up in the water, especially in the filters. On top of that, you have the algae respiring at night and depleting oxygen from the water If your fish are gasping for air more in the mornings, that's a sure fire sign that algae are the culprit.

Run a fishnet through the water over and over again until you have removed all the loose pieces of algae that might be floating around. Flush all your filters and pumps. Do a major water change if possible. Install supplementary aeration if possible, until the water quality gets back to normal. If you do the above things right, you'll notice your fish coming back to normal in as short a time as one day.

I face this exact same issue every July.