Instead of fretting over the price of kelp and nutritional difference between the two types of algae, why not be content with a fine mixture of inorganic salts? A good water-soluble fertiliser such as RapidGro or Peters will consistantly provide all of the elements necessary for healthy plant growth, and it's certainly much less expensive!!!

No, salt water is full of minerals so saltwater photosynthesizers can get what ever there little enzymes desire, freshwater algae have evolved to make due without.