Fish need a heater in an aquarium because they are cold-blooded animals, meaning that their body temperature is regulated by the temperature of their environment. In nature, fish live in a wide range of temperatures, depending on their species and habitat. However, in an aquarium, the temperature can change rapidly and drastically, which can be harmful to the fish. A heater is used to maintain a stable and suitable temperature range for the species of fish in the aquarium. Aquarium heaters are typically used to warm the water to the temperature range that is suitable for the species of fish being kept. This can help to keep the fish healthy, active and breeding. Additionally, a heater can help to prevent the water from becoming too cold, which can lead to health problems such as stress, disease, and even death. Most tropical fish species require water temperatures between 72-82°F (22-28°C) and some species of fish from colder climates may need temperatures as low as 50-60°F (10-15°C). It's important to research the needs of the specific fish you are keeping to ensure that the water temperature is suitable for them.

There are several species of aquarium fish that do not require a water heater to thrive. Some examples include:

• Goldfish: These fish are quite hardy and can tolerate a wide range of water temperatures. They are also able to survive in unheated aquariums.
• Guppies: These fish come from tropical regions, but they can adapt to a wide range of water temperatures. They are also able to survive in unheated aquariums.
• Bettas: These fish are also known as Siamese Fighting Fish, These fish are native to warm, slow-moving waters in Asia, but they can also tolerate a wide range of water temperatures.
• Neon Tetras: These fish are native to the rivers of South America and can adapt to a wide range of water temperatures.
• Hillstream Loach: These fish are native to fast-moving, cool waters of Asia and can thrive in aquariums without heaters.

Please be aware that some of the fish that don't require a heater may still benefit from a stable temperature and a heater can help with that. Since most small aquarium fish are tropical in origin, keeping the temperature of your water consistent is usually necessary. If the tank sits in an environment which stays within acceptable temperature ranges (75-85F), you may be able to skip the heater. However, modern aquarium heaters are small, cheap, and some even come preset so you all you need to do is plug them in and drop them into the tank. It’s one extra piece of hardware, but it’s a small up-front investment that will save you trouble in the long run.