Aquaria Information from



   When discussing salt water quality for our salt water fish and invertebrates, we used to use the term salinity. Now we're asking you to change the term to specific gravity, or the density of salt water compared to that of pure fresh water. 

   Specific gravity is measured with a hydrometer, and is directly affected by water temperature. Most aquarium hydrometers are calibrated for 60o F. Normal salt water has a specific gravity of 1.025 at 60o. When the water temperature rises, the specific gravity will fall roughly by .001 for every 10o. Therefore, at a temperature of 70o, the specific gravity will rise at the same rate. 

   Most marine aquarists will agree, however, that it is a definite advantage to keep marine fish in a slightly diluted environment or at a lower specific gravity. 

   This is because at a lower specific gravity fish metabolism will slow, allowing easier excretion of soluble wastes, and permitting a higher oxygen concentration. Oxygen is less soluble in salt water than fresh water. For fish alone, a specific gravity of no lower than 1.017 is recommended. We advise 1.020 for fish. It has been noted that the occurrence of disease is reduced at a lower specific gravity. Invertebrates should be maintained at a higher specific gravity- about 1.023. 

   Invertebrates are less able to change their metabolism to accommodate changes in their environment. Hence, most parasites, which are also invertebrates, don't exist well in a lower specific gravity. Our specific gravity measurements recommended herein (and on our price list and flier) are at a temperature of 74o.

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