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Two glass bowls
Specific gravity (SG) refers to the density of various ions in the water. These ions are made up primarily of salts and minerals. The more ions in the water (in other words, the denser the ions are packed in the water), the higher the specific gravity. Conversely, the fewer ions present in the water, the lower the specific gravity. Very tiny alterations in specific gravity can make a huge difference in the health of your saltwater tank.
Keep in mind that one of the primary concerns when determining the specific gravity of your tank is making certain that your hydrometer (the device that measures specific gravity) is properly calibrated to begin with.
Lowering Specific Gravity in a Salt Water Tank
Float your hydrometer in your saltwater fish tank and note the reading. For the majority of saltwater fish tanks you are looking for a specific gravity of 1.024 to 1.025 at a temperature of 77 to 78 degrees Fahrenheit.
Lower the specif gravity of your saltwater tank by adding tiny amounts of distilled water until the correct specific gravity is reached, using your calibrated hydrometer. Make sure that the distilled water you are adding is the same temperature as the water in the tank. In place of distilled water you may also use water that has been purified using a reverse osmosis (RO) filter.
Add distilled or reverse-osmosis-filtered water daily to your tank to replace water lost to evaporation. Adding 2 liters of clean water to a 200 liter aquarium (in other words adding .01% of clean water) will reduce the specific gravity of your tank by approximately 1%, or 0.00025. The specific gravity of your tank can be reduced by up to 0.0001 each day, meaning that you can add no more than 8 liters of clean water to a 200 liter tank (replacing no more than .04% of the water in your tank) during any 24 hour period. Reducing the specific gravity at a greater rate could put your fish under stress.
Calibrating Your Hydrometer
Clean your hydrometer by wiping it with distilled water using a microfiber cloth.
Rinse and wipe a clear bowl with distilled water and a microfiber cloth. Make certain the bowl is clean. The bowl must be deep enough that your hydrometer can float in it without touching the bottom of the bowl.
Fill the clean bowl with distilled water (or water filtered by reverse osmosis) and measure the temperature of the water with an accurate thermometer. If the temperature is less than 77 degrees Fahrenheit then set the bowl of water in a larger bowl with warm water (from the faucet) in it. Keep reading the temperature of the bowl of distilled water until it reads 77 degrees Fahrenheit. When the temperature of the water reaches 77 degrees, remove the bowl of distilled water from the bowl of warm water and float your hydrometer in the water. Your hydrometer should read exactly 1.0000 if it is properly calibrated. Note any difference between 1.0000 and the actual reading of your hydrometer. This difference will need to be used later to make adjustments when you read the SG of your tank.