Fish vs. Reptiles
A fish is an animal that lives only in the water, whereas a reptile lives in both land and water. A fish is an aquatic vertebrate that is cold blooded or ectothermic. The reptile is also a cold blooded animal that is covered by scutes or scales. Fish also have scales, although they are absent in cartilaginous fish. They have paired or unpaired fins, while reptiles have small legs for limbs. Fish are found abundantly in the sea or in freshwater. You may see reptiles in water or on land like crocodiles and alligators in the rivers and lizards and chameleons in your house or garden. Some examples of fish are lamprey, sharks, ray fish etc, while lizards, crocodiles, snakes, turtles etc are reptiles.
Reptiles are usually oviparous, that is they lay eggs, although many of the reptile species also give live birth. This is not the case with fish that develop eggs outside their body with external fertilization taking place. The fins of fish are actually modified limbs that have been adjusted for swimming. Reptiles have claws on their feet (except for the legless lizards). Their eggs are amniotic, which makes them suitable for laying eggs on land. Fish breathe through gills which are made out of thread like structures known as filaments. They gulp in fresh air from the mouth and release it through the gills. Reptiles have a solid skull with nasal openings; they have lungs that are ventilated through axial musculature.
In fact, considering the theory of evolution, fish was a dominant form of marine life which eventually moved on to evolve into reptiles and other land vertebrates. The reptiles generally have a three chambered heart with atria, a partitioned ventricle and two aortas. In contrast to this, the heart of fish is a two chambered organ that allows for circulation of blood through the atrium into the ventricle. The excretory system of reptiles is performed by two kidneys which cannot produce urine. Hence, water absorption in reptiles is performed by the colon. Fish diffuse their nitrogenous waste through the gills. Saltwater fish lose water by osmosis.
Summary
Fish lives only in water; reptiles may live in both land and water
Fish have dorsal or ventral fins for locomotion; reptiles have shortened limbs for movement purposes
Reptiles have a three chambered heart, while fish have a two chambered heart
Fish skin is similar to teeth or mesoderm, from where they originate. Reptilian skin is scaly, rough, water tight and irregular