Ceiling fan right? Most cases it is.. There is a insulation seal with-in the casing of the light.. You can use the fan as much as you like.. but as soon as you turn the light on - you have a strong fish smell with in 30 minutes. Take the casing off and replace the seal. (most hardware stores will carry the part but call around first to avoid wasting time) there are a couple more possibilities here. if it is a fan, some of these units have a capacitor in them that will become defective and start to stink up the place. with light fixtures (with or without a fan involved) it is possible that the wattage rating of the lamps is too large and the fixture is "cooking". i have seen this happen to the point of charred crumbling sockets and the insulation/wire looking like a torch was taken to it ... the stink factor is pretty high. I've burned up a lot of electrical devices in my day, and I've never smelled a smell I would describe as "fishy." Before you go replacing and rewiring things, I would recommend simply shutting the power off and giving the offending fixture a good cleaning. Perhaps something got on it that is causeing the odor.