Hi Eikåse
Sorry for the late response. I've been busy working on my YouTube-channel
I dont feed the fish, and I do not think run-off is the problem. But to some extent the biomass from the trees where a problem (not any more ). But I'm pretty sure that the real problem is something else. I found an article that can explain it better than I can in English
"During the summer months, when water is warm, oxygen can be consumed faster than it can be replenished. Ponds and lakes can become "stratified", with warmer oxygen-rich water laying on top of the cool, oxygen depleted water that is found at the bottom of a water body.
Such conditions inhibit function of beneficial bacteria and their breakdown of organics, Muck. Accumulation of organic sediment, muck, increases and adds excessive nutrients for plants and algae to grow. This thermal stratification also makes conditions favorable for the production of noxious ammonia and hydrogen sulfide gases, giving some water bodies a not so pleasant odor.
Vertex® Aeration Systems create a vertical current in the water, using the rising force of millions of small bubbles to entrain the water column, "turning the water body over" and allowing oxygen to be absorbed at the surface.
By moving the lower oxygen depleted water from the bottom to the surface, the thermal stratification will be eliminated. Oxygen levels throughout the water column will increase as will the function of beneficial bacteria. Swings in oxygen levels will be stabilized, preventing fish kills and creating a healthier envirornment for fish."
I plan to install a bottom or diffused aeration systems this summer. Exciting times