AIX

can anyone tell me how to reduce Fs capacity by using echo zero.

Do you mean filling up disk space with useless files? If I understand, then you need to think about another plan. Here is why - sparse files - files filled with "ASCII zeroes" may have some bad effects, such as not using disk space but marking space as used in the directory, Consequently not showing usage for the filesystem as a whole.

What are you trying to do? Please tell us your goal, not how you think it should be done.

Code:

# echo zero
zero
# man chfs

Until we get a reply back, we cannot know what reasonable answer is - yours is fine if the OP wants to change fs characteristics.