What was the pH before it dropped to 6.0?
How long has the tank been set up for?
Was the filter cycled before the fish were added?
What sort of filter is on the tank?
How often and how do you clean the filter?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
Do you have buckets and hoses specifically for the fish tank?
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Assuming the water quality is good and there is 0 ammonia, 0 nitrite and less than 20ppm nitrate, the fish either had a stroke, was spooked and panicked and swam into the glass and injured itself, or had an infection in the brain.
Fish like other animals can have strokes. It's not common but does sometimes happen. They also have heart attack. They can be swimming around normally and just stroke out and die.
If the fish was startled by you entering the room or something else, it might have panicked and bolted into something and caused a brain injury.
If the tank and or filter is dirty, there can be a build up of microscopic organisms that can infect the brain and cause the fish to do weird things and die. Brain infections usually cause the fish to act unusually for a few days before they die, and they don't eat as much or at all during that time.
