It is odd how things come to one during the night, as in when falling off to sleep or waking in the morning. This morning something suddenly hit me about this thread. I was not thinking about fish particularly, but suddenly the photo of that pink foam came into my mind and your comments, and I remembered the initial sudden fish deaths.
Clearly there is something in the water, and I think we have checked most of the usual culprits. Two issues now stand out. First is this pink foam from the plants and the decay of the plant and its smell you mentioned. Can you post a photo of this plant, if any of it is left? Is it a true aquatic? Some plants are highly toxic, and some non-aquatic species can be lethal if they release the substance into the water. Where did this plant come from?
Second issue I now noticed is the filtering of your water. Do you mean by a household softener or similar filter? These can be dangerous because they use salts to soften, or whatever. Let us know more about this aspect.
Still post the video if you can, it may clue us in to something.
Byron.