Pond Boss Forums FISH & FISHING Sunfish-Including Largemouth and Smallmouth Bass Algae and Dying Bluegill ... a connection ?

35% coverage brings 2 things to mind... When algal growth floats to top and becomes matted, it's in the end stages of life when it begins to yellow. If a rain event came that was heavy and cold enough, it "could" sink and finish off the growth process and begin decomposing (burns a lot of oxygen). Algal mats do provide some shading of the water which in turn keeps the temps a bit lower but that could be a double edged sword having that high of percentage of coverage. It does darken the water by not allowing as much sunlight penetration but that can stop or slow photosynthesis to some extent as well as stop some beneficial aquatic plants from growing.
I would highly recommend you physically remove what you can get to so you are effectively taking the nutrients that caused it in the first place. With that much coverage, i'm guessing you don't have a significant amount of aquatic vegetation??
Several things can enhance the algal growth, such as high nutrient inflow, feeding fish, tree leaves and other trash (organic) falling into pond.
As for aeration type, surface and bottom diffused aeration have their place.
For increasing dissolved oxygen throughout the water column efficiently, bottom diffused aeration will be the most effective. Surface aeration is great for oxygenating THAT-the surface, but can be very effective if high biomass (fish mass) is present. It is pleasing to the eye but if running at night, you can't see it and it's really not affecting the entire water column to a large degree. It's also prone to evaporation by atomization, so if run in the daytime, it will increase evap rates. There are cases where both types are used and can be beneficial if run at the correct times.
I think I bought my last water test kit from Amazon.. La Motte water test kit, if you enter those key words it should come up.

https://www.amazon.com/LaMotte-Dissolved-Oxygen-Water-Test/dp/B00BWXMD68/ref=sr_1_31?crid=2Y1WM0ZI2Z6U&keywords=lamotte+complete+pond+water+test+kit&qid=1659940540&sprefix=la+motte+complete+pond+water+test+kit%2Caps%2C102&sr=8-31&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc

Agree - a lot of work but worth the effort !

Check your DO levels if possible.

You could also pull the diffusers to about 3 feet as an emergency if DO gets too low. That will not turn the water over but will put O2 in the upper 3 feet of the water column.

I moved the replies about aerating at night to https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=551612#Post551612 to it's own thread to help anyone in the future that may search for that subject.