if block in python not executed with True statement

I just started learning python with small scripts. I came across a quiz to determine a list to be symmetric if the first row is the same as the first column, the second row is the same as the second column and so on.

def symmetric(block):
    n = len(block)
    i = 0
    for i in range(n-1):
        j = 0
        for j in range(n-1):
            if (block[i][j] != block[j][i]):
                return False
            j +=1
        i +=1
    return True

So the result of

print symmetric([["cat", "dog", "fish"],
                ["dog", "dog", "dog"],
                ["fish","fish","cat"]])

should be False.

However, this code always return True, and in debugger I can see block[i][j] != block[j][i] returns True but the if block is not executed. Is there anything wrong with the comparison or the if block is not correctly composed?