Blackmoor goldfish just died

Question
I had a blackmoor goldfish which lived for nearly a month. I dont know what went wrong with him... one day I saw that my blackmoor is swimming very slowly and had torn fins. I immediatly removed him and put him in a quarentine tank with medicines. I thought that if he had any fin rot disease. He had a good appetite and swimmed very little always sitting in the bottom. His condition detoriates day by day and today I discovered he's no more. I have undergravel filter in my aquarium and I clean it every week. SO here's my question:

Q1: Why did my blackmoor died???
Q2: Was it fin rot??? If so how do i cure it next time if my other goldfish got it???


Answer
When you say, cleaned your tank every week, do you mean take all water out , gravel and clean filter becayse if you did, this is why your fish died.

You just kept re-starting the nitrogen cycle by cleaning the filter and gravel which help the cycle along.


This is why he probably died and the torn fins were probably from the water quality and high ammonia!