Fish interact in a social way. They recognize familiar others and modify their behavior accordingly. Siamese fighting fish will take advantage of a weak fighter by observing fish fights and readily attacking the weaker fish. Fish that clean others act busy when potential "customers' are watching. In a way they advertise.
The myth about fish having no memory or even intelligence is most likely promulgated to justify fishing, when fishermen say it is alright to jab in a hook or gut a fish because they "don't feel any pain".
An example of fish learning occurred when Professor Charles Erikson fed fish after calling to them by saying "fish-fish". When he returned five years later he called to them and some fish came to the surface expecting food. Examples of smart thinkers are trigger fish which use tools to trick prey that hunt them, and frillfin which jump back into rock pools to avoid birds.