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My name is ***** ***** I have been a Vet Tech for 30+ years and a fish keeper over 40 years. What you are describing sounds like the illness took its tool on your Betta. Once they are ill they are very stoic and hide it until it can no longer be hidden. Even with the water change being done correctly, the stress may have been to much for him. Water quality may have been an issue. When we change the water completely we can see Ammonia and Nitrite spikes and with a very ill fish, the immune system is down and the water spikes can shock the system and kill a fish. This is nothing you had done, wrong but the underlying illness that you described. I suspect there was a water quality issue that caused the original illness and fin rot. This is very common in Bettas, since most people do not use filtration or areation. I hope that this information puts your mind at ease. If you would still like to post the pictures, I can look at the, but pretty sure the death was the result of a Water Parameter Spike. If I have addressed your concerns, Please Rate my Service 5 Stars.
Please post the pictures. I suggest the API Master Test Kit. It is the most reliable and will give you true readings.
The Ammonia should be 0, Nitrites 0 and Nitrates 20 ppm or less. PH 7.0. Never do a full water change. Only artial at 50% and then 25% after that. The tank has to cycle.