Hi, Our bathroom sometimes smells fishy as well; it turned out to be the sink drain. Our water guy told us to pour bleach in the drain once a week (1/2cup) and let it sit 30 mins. and then flush with water.
Another thing we tried was to mix vinegar and baking soda and let that sit in the drain 30 mins and then turn on faucets for a minute.
This might have nothing to do with your problem, just thought i'd add my two cents worth.