This is completely normal as the smell is a natural by-product of living marine phytoplankton growth, known as Dimethyl Sulphide, and it is one of the components that makes up the "smell of the sea".
Hobbyists will sometimes note that their own cultured phytoplankton doesn't smell as much, well the answer is quite simple; home cultures are rarely grown to the densities required for the smell to be noticeable (there is simply not enough algae in the bottle), and they might be growing a species that does not produce a large amount of sulphide.
Our strong blends of 7 concentrates are more prone to releasing the sulphide as they contain more than one species of alga. Phytoplankton products that have been concentrated will also be producing more sulphide due to the sheer amount of algal cells present.
This is the most common question we have about this product and urge you to continue to feed within the 6 week shelf life period.