One of the things in this hobby that fascinates me the most is the amount of symbiotic relationships that can be observed in the tank.
The example above is the cleaner shrimp and pretty much any fish. The fish allows the cleaner shrimp to eat parasites off of it. The shrimp gets a tasty meal.
Then there is the clown fish and anemone relationship. The clown fish gets protection from the anemone and the clown fish feeds the anemone food scraps. This isn't occurring in my tank but maybe it will some day.
Then, of course, there is the relationships between corals and the zooxanthellae algae that lives inside of them. The algae gets a place to live and the coral feeds off of the photosynthetic byproducts of the algae.
Probably the most basic relationship is between the denitrifying bacteria in the tank and the animals. The animals produce ammonia as waste which is then broken down to nitrite and later nitrate by bacteria in the tank. The bacteria get to thrive and in tern the animals get cleaned water and also thrive.