Fishes' own taste
Protein, and especially hydrolysed free amino acids, betaine and other molecules resembling amino acids, are substances that have a strong chemical attraction on fish.
A distinction is made between the L-form and the D-form. Fishes do respond to L-amino acids but not to D-amino acids.
In order for a flavour enhancer to work, a certain concentration of free amino acids has to be reached.
In general, herbivorous fishes are attracted by free amino acids from vegetable protein and carnivorous fishes are attracted to a combination of free amino acids that occurs in animals that live in water.
Having an own taste is important for quickly recognising and quickly taking in the right feed. This is nice for anyone who keeps fishes, as it looks quite attractive. But it is also important for the fishes themselves. A quick intake of feed prevents the loss of ingredients that dissolve in water, such as vitamins.