Given the large amount of waste produced by this fish and the very small amount of water being changed, it would appear that the water parameters are not on the healthy side. Of course I can on speculate since the water has not been tested. Changing somewhere between 25-40 gallons a week might be more appropriate. Maybe even more than once a week. Testing the water for ammonia, nitrite, nitrate, and pH would be helpful.
The fin rot or fungus on the tail may have been brought on by stress leading to an impaired immune system caused by poor water conditions.
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