Do you need to have rocks in your fish tank?

Although pet shops may sell them, Rock pebbles are probably not a good idea in any aquarium because food bits and 'Poo' that falls to the bottom, drops and slides between the pebbles and then the food bits can not be eaten by the fish or the Poo be collected or even syphoned out. So the rubbish stays in the pebbles and fouls/poisons the water. Fairly fine river gravel is the best substrate for most aquaria. Plants can grow in it and any mulm or rubbish tends to sit on top where it can be seen and easily removed by syphoning.