Hi everyone,
Recently two megalechis thoracata I have kept for at least 3 years have died one after the other in sudden deaths. I am baffled as to what has killed them so I'm hoping that anyone here would have any idea.
Symptoms: blood streaks in fins, jagged edged fins. Both fish were still feeding well before their death.

Upon seeing the ragged fins and blood streaks during a water change, I dosed melafix and pimafix to counter what I believed to be finrot which was caused by bacteria. I underdosed as I was afraid that the species of catfish may be more sensitive to medication than other fish. The next morning, I found one of them(more severe symptoms) dead and by evening time the other has died as well(with only minimal ragged fins). The other fishes are doing fine including armoured catfish, angelfish and loricariidae catfish.
Tank specs:
3x1.5x1.5, 180l
Ammonia 0
Nitrite 0
Nitrates 10ppm
pH 7.4
Weekly water changes of 70-80%

The two fishes are gone but I would like to know what killed them since they were with me for quite some time. Is it the bacteria or was it because of inappropriate medication? If anyone could offer me insight I would appreciate it.

Thanks in advance