Fish can generally go for about 2 weeks without food. However, this greatly depends on the type of fish, their size, and the environment conditions.
Fish are relatively low in metabolic needs compared to other types of animals. This allows them to survive for extended periods without food by slowing down their bodily functions and reducing their activity levels.
Some fish, such as goldfish, are capable of surviving without food for upwards of a month under certain conditions. For example, if the water temperature is cooler it slows the fish's metabolic rate and less food is required for survival. Furthermore, goldfish can also tap into their body fat reserves and other tissue for energy, thereby prolonging their survival.
However, it's important to understand that survival doesn't mean that the fish are thriving or even maintaining their health. Extended periods without food can lead to nutritional deficiencies, which can weaken them and make them more susceptible to disease. Not feeding your fish for an extended period could also trigger aggressive behavior, as starving fish may get desperate and turn to cannibalism.
In a natural habitat, most fish are opportunistic feeders, meaning they eat whenever food is available. In a home aquarium, it's advised to feed most fish once or twice a day. Overfeeding can be as harmful as underfeeding, it can pollute the water and even lead to excessive growth.
Additionally, while adult fish can typically handle going for a few days or a couple of weeks without food, this is generally not true for young fish. Juvenile fish are growing rapidly and need a constant supply of nutrients. A few days without food can be detrimental for a growing fish.
Moreover, some fish species with high metabolisms such as 'marines', can't survive without food for more than a couple of days.
It's essential to feed your fish a balanced diet and keep them in a suitable environment to ensure they live a healthy and long life.
So, while it's impressive that many fish can survive without food for around two weeks or even more, it's not a situation they should regularly be put in. Proper feeding and care are vital for a fish's overall health and well-being.