Good to see your DO reaches a more acceptable level late in the day but you should also measure ammonia late in the day as opposed to the AM. As for your "Nitrates", can I ask what type of test kit you are using? I think you may be measuring TAN Vs "Nitrates"? Either way, your water seems to be cycling healthy and the increased aeration seems to be helping.
I am with Mike and others on this one, it is either a plankton or pollen. Looking in your area for pollen contributors, it seems Ash trees are the main contributor this time of year.
https://www.stlouisco.com/Health-and-Wellness/Pollen-and-Mold-Center
