Ben9108 wrote:

I would be grateful if someone help me to correct sentance. The polluted water has not been made by poor tank condition as all fresh water tanks have been clean last month.

The contaminated water has not been caused by the unclean condition of the tanks because all the tanks were sanitized just last month. (This sounds quiet awkward and loigcally weak for the point the statement tries to prove, although it's understandable)

Here is how I would say it:

The contaminated water must have not been caused by the alleged “unclean condition” of the tanks because all the tanks were sanitized just last month.

Comments: In this context, [contaminated] is better suited than [polluted].

“Has been clean” is in correct in that usage. You can say "he has been clean for 2 years from alcohol", but not in this context.  If you must use [clean] in a passive voice, you need to used the past participle form [cleaned].

Referring back to the context of water tanks, [sanitized] is my choice word because it offers a deeper meaning of just cleaning.