In freshwater environments, fish have lower osmotic pressure compared to the water. This means that they tend to lose water to the environment, which is replaced by actively absorbing ions and drinking water. Freshwater fish are adapted to regulate their internal salt levels in this way.
When placed in saltwater, the environment has a much higher salt concentration compared to the fish's body. This creates an osmotic gradient where water moves out of the fish's body into the saltier water. As a result, the fish loses essential water and electrolytes, leading to dehydration and potentially fatal consequences.
Furthermore, the high salt concentration in saltwater also affects the fish's cellular functions. Enzymes and proteins within the fish's body can be disrupted, leading to organ damage and malfunction. Additionally, saltwater contains different ions compared to freshwater, which can interfere with the fish's ability to maintain ionic balance and regulate other physiological processes.
In summary, placing a freshwater fish in a saltwater aquarium subjects the fish to severe osmotic stress, dehydration, and disruption of cellular functions. This combination can rapidly lead to the fish's death, making it dangerous to introduce freshwater fish into saltwater aquariums.