a strange fishy smell is most often caused by an overheating electrical accessory, such as a socket, switch or junction box
apart from electrical faults, it can also happen if you have a high-wattage lamp in a lampholder that is designed for lower power, or where a lamp is enclosed e.g. in a dowlighter and the heat can't get away/
if you turn off the power for a few hours and the smell goes away, you have a clue. if the smell occurs in winter it may be to do with a heater; if in the summer it might be with a fan or air con; if at night it may be lighting; if when cooking it may the stove.
to find out what a hot electrical accessory smells like, buy a cheap electrical item such as a brown junction box, and put it on the stove. They are made of thermosetting plastic which does not melt when it gets hot, but it discolours, chars and smokes. See if the smell is the same.
if you can't find an overheating electrical accessory, there might be a junction box hidden in a wall or ceiling; if it has a poor connection inside it, that can cause overheating. Here in the UK junction boxes are not permitted to be hidden in walls or ceilings in case this sort of problem arises. An ionising Smoke Detector alarm may go off if placed close to the source, or above it.