Why do the eggs from one of our hens always smell fishy?

If this is the only symptom you're seeing, she's probably not sick. (If she does show other signs of illness you will want to take her to a licensed avian veterinarian for a firm diagnosis and treatment options.)

Interestingly, in some hens that lay brown eggs, eating too much canola or rapeseed meal can cause a fishy smell in the eggs . Not all hens are affected by the process that causes the smell. The smell is caused by the accumulation of trimethylamine (TMA) in the yolk. Most hens metabolize the TMA into another (odorless) compound, but brown egg layers don't do that as efficiently, so when feeding canola meal, in some cases you may end up with--ick--fishy smelling eggs. Your hen may have the chicken version of bad breath: not halitosis, but hal-egg-tosis! ;-)

While there's nothing you can do about the way your hen digests her food, the easy remedy would be to change the feed you're offering your flock to a feed that doesn't use canola or rapeseed meal. High levels of canola meal in the feed is also related to reduced feed intake, reduced growth rate, small egg size, and some other issues. If your poultry feed contains canola, keep in mind that feed producers do try to balance the use of canola meal with other, higher-quality ingredients so most people probably won't see these problems.

If you are having a problem with the fishy odor, though, it would probably be best for you and your hen if you switched to a feed without canola.