1) If i am filling a bucket from a tap which is connect to a tank of 1200 liters, can i consider that bucket water as qur water and all the laws of qur water applies on that as well.
2) If I came to know that I was in state of janabat after one day, do I need to repray all my namaz or its ok?

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The water in the bucket would not be kur just because of where you got it from, but rather the size of the bucket is the determining factor
Second, yes if you discover on second day and u know for sure it was from day before then you would need to repeat the prayers