Had a similar problem recently. It occurred right after I changed feeds. I read another thread on this site with links to several studies indicating that certain brown egg laying breeds (like Rhode Island reds) sometimes have a "defective" gene that prevents them from being able to metabolize certain foods, (like canola meal) the favors of which wind up in their eggs.
If that's too much info, just switch to a feed without flax seed or canola meal and your problems will likely go away. Worked for me. I Switched to a hideously expensive organic feed and voila! no more fishy eggs.
Someone on the other thread also noted that the fishy egg syndrome could be seasonal. I noted that in my case the chicken responsible was molting. when they molt I understand their bodies need protein to replace the feathers. Since canola meal was one of the main sources of protein in the feed I switched to, it would stand to reason that molting would increase the likelihood that canola meal feed would produce more fishy eggs.
Maybe I'm over thinking it. But then, that's what I do.
Bottom line, better feed makes better eggs.... And happier hens.
I would love to know the answer to why eggs smell like fish! Have had chickens for 6 years and never had this problem until now-cracked one the other day and thiught i smelled it but it looked fine-thought it was just in my head until i tasted it-horrible! Completely grossed out. Have always used the same feed. The only thing that is different that i've never had is that one chicken hatched a chick that is about a month old-dont know why this would affect anything but nothing else is different. Any help appreciated!