It could be ammonia in the tank that is causing it.
Is your fish population still the same as before ???
Sometimes one can die behind a rock and go unnoticed.
But definitely a water change is in order. Just make certain the water is correctly dechlorinated and that it is at the right temperature when adding it.
Lastly I have noticed that blue green algae (seems like long tough strands of algae but are in fact Cyanobacteria) can have a strong smell that can be perceived as a fishy smell. Remove any that you find and srub rocks that have it. Cut back on feeding.
Let us know how you go.