1.) Diffusion systems are not good aerators. Their main job is to destratify a pond by moving water using air, exposing it to air on the surface, but aeration is not as significant as an actual surface aerator. They are great for ponds and are very efficient at what they do, but they are not designed specifically for aeration. They will actually bring your entire water column to, or close to, late spring and summer surface temps, which is bad for trout once water temps reach 70 degrees. Optimum temps for trout are 55 to 65 F. Once your water column reaches 70 and keeps climbing you can kiss your trout goodbye shortly.
2.) Your trout would normally die this time of year regardless of whatever aeration you use, due to too high of water temps. Even surface aerators will, although producing oxygen, warm the water.
3.) If you did not mix you water column with the diffusers you could have bought a little time for your trout (cooler deeper water refuge), but your cooler water would eventually become too low in dissolved oxgyen especially in an old pond such as yours.
3.) Consider yout trout a bonus fish for fall, winter, and early spring unless you take measures to keep water temps low enough year around. This would most likely require aerated well water which can be expensive, and is required in copius amounts depending on the size of the pond. If you choose to do this, small is better, as it requires less well water. However your warm and cool water fish will suffer growth loss if you want them also.
And finally yes your diffusers most likely brought up some lower D.O. water, especially if you started up the system to late or didn't pulse it. The darker water may be due to an algae bloom crash, or your diffusers may be bringing up mucky sediment which in a pond as old as yours may also reduce D.O. Sometimes diffusers stir up the bottom at start up and sometimes they are installed incorrectly or maybe even upside down.
There are some of us here that grow trout out year around in ponds. But you will find our ponds receive well water or spring water, are fairly small, and if we use diffusers we only turn them on when air temps are lower then the optimum trout temps, such as at night. My trout pond is only about 90 by 45 feet and although only 8 feet deep it has very steep sides to keep warming down. I run about 41 gpms of aerated well water into the pond 24/7 from May to the end of October. Even then my surface temps will climb into the mid 60's when we have days when air temps climb in to the 90's. But once the sun gets low in the sky the temps start dropping again.
