Check your curtain rods. I hear ex-wives tend to leave dead fish in there.

rozilla - LOL!!! That was my first thought when I read this post. For those of you who haven't heard this story, see the link below.

Here is a link that might be useful: Fish smell in house

LOL That's the first thing I thought of too! We had the same problem only it was localized in one area of the kitchen and only when I vacuumed. My husband and I are allergic to fish so it couldn't be cooking odor. I thouhgt that maybe the cat had thrown up in that area and it had somehow gotten under the flooring (we had a laminate floor). We recently had to replace our kitchen floor and since the flooring was up we replaced the sub-floor where the odor was coming from. I started vacuuming the new sub-floor to get up any loose particles before the floor was put down and it started to smell! It wasn't the vacuum because I have different ones I use in the kitchen and they both caused the smell. We put the new floor down and I almost cried when the smell continued. Then one day it was gone. I don't know where it came from or where it went but I'm just grateful it's gone. I wish I could offer some advice but at least you know you're not alone. I hope yours disappears like mine did!

Do you have a crawl space under the house?

This sounds strange, but an electrical problem can be the source of a fish-like odor. Google "fish smell" and electric and you'll see quite a few hits.

Some wood smells really bad, maybe they used it as a subflooring. Sorry you are having this problem.