Planted Tank Obsessed
Join Date: Dec 2012
Location: In an aquarium nearby
Long story short, water osmosis phenomenon means water molecules will tend to move to a denser liquid from a less denser as to dilute the denser one. The speed with which this phenomena happens is called osmotic pressure. The greater density difference the larger the osmotic pressure.
All fresh water organisms are hyper osmotic, meaning they have denser fluids inside of them as opposed to the surrounding water. TDS controls the surrounding water's density. The larger the TDS the higher the density. All aquatic organisms have ways of regulating the density of their internal fluids to some degree. This process is called osmoregulation and it's performed by all fresh water creatures, as far as I know.
This osmoregulation process has a low and high watermark and it's specific to individual species. Because of that, you will need to maintain the TDS to a specific range for the specie you are keeping.
What happens when the TDS is lower than the low watermark of your creature's osmoregulatory system: water molecules from the surrounding environment will create an osmotic pressure and actually "push" into the creature making the osmoregulatory system work extra to try to compensate the dilution of internal fluids.
When TDS is too high, the reverse is happening but this is slightly easier to compensate and causes less damage.