There's something wrong with this sentence but I can't put my finger on it!!

The federal government has put into effect a mandate that requires all gasoline in Canada to contain 5 percent ethanol.

Maybe another "that" after "requires"?? The "...that ____ that..." bothers me though. Too many 'that's!

and if another "that" was added then "to" would change to "must", I guess?

The sentence looks fine the way you typed it the first time. You don't need another "that" in this sentence.

The only thing I'd do to the original sentence is to remove "to" before "contain."

The sentence you provided appears to be grammatically correct and free of any obvious errors. However, if you suspect that there is something wrong with it, it might be helpful to consider a few potential areas of concern:

1. Clarity: Verify whether the sentence accurately conveys the intended message. Check if there are any ambiguous or misleading phrases that could be causing confusion.

2. Accuracy of factual information: Ensure that the information stated in the sentence is factually correct. Double-check whether there is indeed a federal mandate in Canada requiring gasoline to contain 5 percent ethanol.

3. Context: Consider whether the sentence fits within the larger context of the passage or discussion. It may be necessary to review the preceding or subsequent sentences to identify any inconsistencies or issues.

4. Formatting: Examine the punctuation, capitalization, and syntax of the sentence to ensure that it adheres to proper writing conventions.

If none of these areas raise any concerns or red flags, it is possible that the sentence is correct as originally written.