Fish survival times without feeding can depend on several factors, including their species, age, size, overall health, water temperature, water quality, and the availability of naturally occurring food in their environment.
Broadly speaking, healthy adult fish can often survive anywhere from a week to a maximum of two weeks without eating any artificial or provided food. But keep in mind that survival is not the same as thriving. Going weeks without eating can likely cause some stress and health issues for the fish. However, many types of fish are well adapted to infrequent meals and can even eat algae, bacteria, and small invertebrates that exist naturally in their environment, helping them to sustain themselves during periods of food deprivation.
Fish physiology adapts to survive periods without food. When they don't eat, they become less active in order to conserve energy. Also, fish bodies can start to consume their own tissues for sustenance when deprived of food for longer periods. It's more like a short-term survival technique rather than a sustainable lifestyle.
Young and smaller fish usually run higher energy costs, so they can starve faster and may not survive more than a few days without food. Large and mature fish, on the other hand, have larger fat reserves that can be used for energy, allowing them to survive longer periods without feeding.
A commonly shared belief is that fish in cooler waters tend to be able to survive without eating for a longer period than those in warmer water. This is because cooler temperatures slow down the fish's metabolism, reducing the energy and nutrition they need.
It's important to note that starving a fish is wholly dissimilar from fasting a fish. Generally, if a hobbyist knows they will not be able to feed their fish (for example, during a vacation), it would be safer to withhold food for a week while providing a clean environment with optimal water conditions, rather than relying on automatic feeders which can sometimes malfunction and overfeed, thus polluting the tank.
However, prolonged periods without suitable nutrition can lead to organ damage, hindered growth, weakened immune system, and ultimately, increase their susceptibility to diseases, possibly leading to their death. Thus, it's crucial to ensure that fish are well-fed regularly and are getting proper nutrition. But be careful not to overfeed as it brings undesirable consequences such as water contamination and digestive issues. It's all about maintaining a balanced approach.