You Are Right
That is what happen + nitro spike as well.
I had continued to care for them as usual for years, however, I knew they were obviously getting very large and of the impression increased and frequent water changes, would be enough. I didn't know about these canister filter systems. Everything that could go wrong went wrong, really fast.
When they started showing stress in the pass, a quick water change and everything went back to normal. Not eating was a new symptom, though.
The filter system must have obviously been on overload, the size of fish, tank size and inexperience.
Watching in desperation as it was all failing.
A very sad experience.
Thank you for responding.