Osmotic pressure is the hydrostatic pressure produced by a difference in concentration between solutions on the two sides of a surface such as a semi permeable membrane or cell (Ed: or for instance the cell on the gill of a freshwater fish and the surrounding water). Osmotic pressure also occurs in and around freshwater fish as they must keep on osmoregulating . . . spending much energy retaining salts and excreting water. ... What makes matters worse, though, is when aquarium salt and other additives are added to the aquarium . . .not only does the conductivity increase but now the ions that were in balance in the natural water (tap water or water source) are completely out of balance. To explain this better ionic imbalance occurs when Chloride and or Sodium become the major cations or anions in water. then it becomes imbalanced and unnatural. If you look at just about all fresh waters with very few exceptions it is calcium and or magnesium that are the major cations and sulfate, alkalinity/bicarbonate/ carbonate are the major anoins. Even in Lake Tanganyika which is the biggest exception to the freshwater rule the principle cation is magnesium and the major anion is bicarbonate/carbonate. These ions even though they are water soluble they are much less soluble than the chloride and sodium ions. All aquarium additives have one thing in common, included in their products are chloride and sodium. It is the chloride ion then the sodium ion that are taken in by the freshwater fish due to there water solubility. When these ions are so numerous and are the chief cations and anions in an aquarium, the freshwater fish is under much stress osmoregulating thus spending much energy as chloride and sodium are being taken up.
thanks mfk
