I have worked in a clincal lab setting for sometime and there are many processes occur that are complex yet amazing.
In mamals we move the salt around our bodies thru osmotic pressure gradients in the kidneys and gradients in tissue versus whole blood. Sea animals have a very big problem they live in a solution that has almost the same osmotic levels as their blood to pump waste out of their body requires alot more energy to complete this is why Saltwater fish need alot more food to burn than say a fresh water animal . to maintain waste excretion thru their metabolic pathway.
Thats why some saltwater fish need to eat 4-5 times a day to be healthy where a freshwater fish can go longer.
This creates a greater waste load on the tank for a small animal
the more animals the more waste or they may starve if you don't feed .
here's example of energy needs od freshwater versus salt
4.2.1. Diffusion and Osmosis
Diffusion: Movement of molecules from a higher concentration to a lower.
Osmosis is diffusion through a semi-permeable membrane
4.2.2. Freshwater fish
Freshwater fish are hypertonic in comparison to their environment.
Water passes through the gills and ions diffuse into the water
They have to eliminate water constantly from their body
In freshwater fish with large lesions on epithelial layer or gills, fluid therapy is contra-indicated.
Most important task of the kidney is to eliminate excess water
4.2.3. Saltwater fish
Fish in the marine environment are hypotonic and lose water at a high rate.
They have to drink seawater and excrete minerals
Even when fish rest, they have a high energy requirement due to the ATPase dependant processes.
Most important task of the kidney is to eliminate excess electrolytes (Mg
Some species have aglomerular kidneys
here's wher I borrowed that text from..