Fish odor is a strong, pungent smell that is difficult to get rid of, especially when the odor has set into carpeting. Commercial carpet deodorizers are somewhat effective, but are often expensive and contain harsh chemicals. Instead of using commercial products, use natural carpet deodorizers that are safe, inexpensive and just as effective.
Here's how:
Sprinkle a thin layer of baking soda over the entire carpet. Let it sit for two to three hours, or overnight if necessary, until the fish odor has diminished.


Vacuum the carpet to collect the baking soda. The baking soda will have absorbed the odor, pulling it out of the carpet and eliminating the odor.


Fill a spray bottle with an even mixture of distilled vinegar and water. The vinegar will naturally kill any bacteria that may be the cause of the odor.


Spray a light mist of the vinegar over the carpeting, and allow it to air dry. Repeat four hours later if any fish odor remains.

source:ehow.com