What would cause a fish-like smell to come from the shower unit even when it is switched off?
I have that problem too. The smell comes from mildew growing on
the sides of the drain pipe above the waterline in the trap. It's
rather dirty in there below the drain plug, and you never really
clean it. When you turn on the water, it disturbs the mildew and
releases bad smells. The water trap in the u-turn under the sink
keeps sewer smells out but this is before that. The solution is to
grab a bottle of bathroom mildew remover and spray it heavily
around the drain plug to saturate all sides of the drain pipe. Try
this once every few months to keep your bathroom smelling fresh.
Don't forget the shower and bathtub drains aswell, and kitchen
disposal too if you have a problem there.