Here's why:
* Freshwater fish are adapted to low salinity: They have evolved to thrive in water with very low salt levels. Adding salt to their environment can cause serious health problems.
* Salt stress: Salt can disrupt the delicate balance of electrolytes in their bodies, leading to dehydration, osmotic imbalance, and even death.
* Toxicity: Salt can be toxic to freshwater plants and invertebrates that may live in the tank.
Exceptions:
* Quarantine: In some cases, a very dilute salt solution (around 1 teaspoon per 5 gallons) may be used for short periods during quarantine to treat minor infections or parasites. However, this should only be done under the guidance of an experienced aquarist or veterinarian.
* Specific species: A few rare freshwater fish species, like those from brackish water habitats, may tolerate slightly higher salt levels. However, it's important to research the specific needs of your fish before adding any salt.
Always consult a reputable source or a veterinarian before adding any substances to your freshwater tank.