I don't believe the original poster has responded on some key questions, in particular, IMO, the use of kitchen sponges etc.
Clearly the tank had some serious issues to begin with - overstocked and a major algae problem. Not trying to be mean here, just an observation.
There are any number of issues. I don't necessarily believe that the tap water is the cause, even if there was a chloramine spike at the time. It was 8 gallons out of say 100. Low concentration. Same is true of the windex though IMO it is not a good plan to have around.
I am interested in anything possibly on the hands. anything else going on at the time including other house cleaning, painting, etc. A death of this magnitude, which also killed inverts, requires a significant immediate toxin. IMO, not ammonia.
It would be good to know about the behavior of the fish as they were going. And if anything else has been put in the tank since then and survived. I would be interested in what algae stuff was used an how much.
I suspect to some degree this may have been a "perfect" storm scenario of several smaller things - not too good - that came together at once. But it will be important to eliminate certain toxins as it may kill further fish.
How deep is the sand bed? There is concern if indeed there was algae growing on the surface that it was some sort of hydrogen sulfide issue though in a 2 month old tank I would be somewhat surprised.