But now you've not learned what happened and can step into the same do-do again.
How was the pool filtered? If I had to guess, they died from ammonia and nitrite poisoning, likely low levels and long term.
IIRC, you don't measure water parameters. You'll tend to kill more fish until you do. Just MHO, bro. I used to not measure too then got tired of killing fish cuz I stock heavy. You do too it seems. With heavy stock, you must measure water especially at the very first sign of ANY appetite or health-related problem.
The RTC in the photo looks female to me. Females grow substantially smaller than males, or so I see at least with my RTCs.