Reptiles can live both on land and in water, and fish live solely in water. Differences in reproduction, body structure, waste elimination and methods of movement are significant as well.
Fish can live in both freshwater and saltwater. Reptiles are capable of doing the same, but will always have nearby habitation. Reptiles prefer places like rivers that have an active stream. Rivers provide food, while also supplying shelter for reproduction. The reproduction cycle of reptiles is typically oviparous, they lay eggs. Because of the reptiles ability to navigate easily on land, eggs are usually inland away from predatory danger.
The number of chambers in the heart of reptiles and fish is a significant difference in terms of evolutionary theory. Fish have two chambers and reptiles have three. This difference illustrates the scientific theory that fish eventually evolved to move onto land. The mutation in the heart gave early reptile ancestors the ability to move on land and out live kin that did not exhibit the heart mutation. Eventually, the mutation became a defining characteristic in the species. Fish, which still remained in the water, continued on with a two-chambered heart. In water the additional chamber in the heart offered no genetic advantage.
Fish also have gills for breathing. Reptiles have developed nasal passages and lungs.