[lilwigs]

What do these guys mean?


<LI>The ability of water to dissolve, combine with, or suspend other elements and compounds can be helpful as a supply of necessary nutrients. Water can also be a source of trouble for the aquarist because it can contain chemicals that are irritants or toxic to aquatic organisms. Aside from overall stress, water quality is the most common cause of fish diseases. ¹

<LI>A constant level of minerals in the water is necessary for aquatic life. Changes in the amounts of dissolved solids can be harmful because the density of total dissolved solids determines the flow of water in and out of an organism’s cells. Concentrations that are too high or too low may limit the growth and may lead to the death of many fish or reefs. ²

A level of 400ppm or less is recommended for most freshwater fish although many softwater fish demand a significantly lower level. Conversely, saltwater fish require a very high level of anywhere from 5000 to 50,000 ppm. Total Dissolved Solids are also important for proper osmotic regulation, which is the relationship of water versus dissolved solids in the cells and the external environment. The greater the amount of solids in the water versus the solids in the tissue of the fish will result in a fluid loss via the gills. ³
http://tdsmeter.com/abouttds_aquaculture.html

[kevinpo]

The part you have highlighted is in reference to the salts and other minerals that are contained in saltwater. Such as salt, calcium, magnesium, carbonates, etc, etc.
Most salt mixes contain the solids in the proper ratios so it is best to start with freshwater as close to 0 TDS as possible so that you don't skew the ratios over time.

Note: Always add the salt to your water and never the water to the salt when mixing up a batch of saltwater. If you add the water to the salt you can cause some of the elements to plate out (no longer part of the TDS).

HTH,
Kevin