Here are a few possibilities you might be thinking of:
* "Dead fish smells the worst when it's nearest." This means that the closer you are to something unpleasant, the more noticeable it becomes. It can be used to talk about problems or negative situations that are easily ignored when they are far away, but become overwhelming when they are close.
* "A dead fish stinks from the head down." This is a similar idea to the first one. It means that corruption or bad behavior starts at the top and spreads downward. It's often used to criticize leaders or organizations that are perceived as being corrupt or ineffective.
* "The fish stinks from the head." This is another variation on the same theme, meaning that bad leadership or a corrupt system is the root of problems.
It's important to remember that idioms are often specific to a particular language or culture. If you're unsure about the meaning of an idiom, it's always best to check a dictionary or ask a native speaker.