What organ eliminates waste and helps maintain water and salt balance in fish?
Wastes can pass out of the body of a fish via its gills. However, fish also have kidneys to filter the circulating blood and to remove the toxic waste ammonia. Kidneys help to control water balance inside their body.
In fresh water fish, there is a danger of inward osmosis because of the greater concentration of fresh water outside their body relative to inside. Gills in this case will take in salt and water will enter the body. They will excrete urine which will be dilute and this will remove excess water that might otherwise cause their cells to swell and possibly burst due to the environment they live in. This is known as being in a hyptonic solution.
In salt water fish, they have the opposite situation---there will be a greater quantity of fresh water inside the cells relative to the salty environment. Therefore, outward osmosis is a possible problem and their cells could shrink as they lose water by plasmolysis. They could also gain salt but in this case their gills help to excrete the salt from the environment. This helps to regulate their water balance. This is an example of what occurs in a hypertonic solution.
By having adaptations like gills and kidneys, fish can survive in different habitats.