Hi and welcome to the forum
Normally we prefer it if you start a new thread. It makes it easier to follow. However, since we are here.
If the fish started developing problems after you did a water change, there was probably something in the new water. Do you dechlorinate new water before adding it to the tank?
If the water company did work on the pipes recently, they would have increased the chlorine/ chloramine levels in the water and if you add straight tap water to the tank, the fish might have been poisoned by chlorine.
All water should be free of chlorine/ chloramine before it's added to any aquarium or pond containing fish and or other aquatic organisms.
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How often do you clean the filter and how do you clean it?
Ammonia that is 0.8ppm should not be a big deal in water with a pH of 7.0. If the pH was higher (say 7.6+), then it would be an issue. But at 7.0 it shouldn't be a problem.
That level of ammonia could be from the dead fish.
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Any chance of a picture and short 20 second video of the remaining (and or dead if available) fish and tank?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
If the video is too big for this website, post it on YouTube and copy & paste the link here. We can view it at YouTube. If you are using a mobile phone to take the video, have the phone horizontal so the video takes up the entire screen. If you have the phone vertical, you get video in the middle and black on either side.