Smell like fish oil coming from cupboard with hot water tank
by fish oil smell, do you mean a smell of sulfur? if so, it could be that the pressure relief valve is opening or the tank is leaking in some manner, and the heated water is releasing a sulfurous odor. look for any signs of leaking water.

a common place for a water heater to leak is the pressure relief valve. if the relief valve is leaking or opening, check the water pressure and the thermostat. high water pressure can force open the relief valve and a thermostat set too high or malfunctioning can cause over pressure. if there is a one-way valve near the inlet side of the water heater, that could prevent the water from expanding back and forcing water out the pressure relief valve.