Measuring Specific Gravity (beginners guide)

Reef tank containing fish, corals, and rock.
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    So what exactly is specific gravity in a reef tank?

    In simple terms, specific gravity is a measure of the density of a substance in direct comparison to the density of water itself. Specific gravity is also known as relative density. When we talk about specific gravity in relation to a reef tank, we are talking about the relative salinity (number of dissolved salts) in the reef tank’s water. In very simple terms…how salty the water is!

    How can we measure specific gravity?

    Most reef tank owners will user a refractometer or hydrometer to measure the specific gravity. A hydrometer typically consists of a swing arm located in a plastic or glass casing. The swing arm moves to display a reading once the case is filled with water. A refractometer works by adding a couple of drops onto a glass prism, as the light bends as it passes through the prism which is subsequently focused on a visible scale.

    Refractometer or Hydrometer?

    The recommendation is to definitely choose a refractometer over the less-accurate hydrometer. Readings with a hydrometer can often be inaccurate. This can be caused by a number of factors including build-up on the swing arm and air bubbles attaching to the swing arm. Both of these factors can cause inaccurate readings. However, if you are on a tight budget, it may be worthwhile pursuing until you can afford a more expensive device. A refractometer is considerably more accurate, and consistent with it’s readings. It can also be recalibrated by testing water with a known specific gravity such as RO/DI water that should have a specific gravity of approximately 1.000.

    How much is a Refractometer or Hydrometer?

    At the time of posting, a hydrometer should cost around $10 USD, and refractometers will cost between $15 to $25 USD. If you search for hydrometers and refractometers you will also see a range of electronic devices that perform the same functions as the manual ones. However, electronic devices do vary considerably in quality(and ultimately accuracy). Certainly worth researching and reading the reviews before purchasing.

    What do I do with the results?

    Keep a note every time you take a reading. Keep a traditional log book, or there are some great mobile apps out there that will help you record the information(along with the other water parameter readings). With this information, you can determine trends in your specific gravity and be proactive to ensure the right level is maintained.

    How important is all this specific gravity stuff?

    A consistent level of specific gravity in your tank is extremely important to the health of your reef tank and it’s inhabitants. Some fish, coral and invertebrates are only happy with a specific range of water parameters, and so getting the right level is very important to their health. Sharp swings in salinity can also have a negative impact on your tank’s inhabitants, so a consistent level of the right salinity is key to the health of your reef tank.

    What are the recommended Specific Gravity levels?

    Rather than a definitive specific gravity level, there is a known range that most reef tank owners have the most success with:

    Reef Tank 1.023 to 1.028
    FOWLR(fish only with live rock) Tank: 1.020 to 1.025

    These are only the typically suggested ranges, so do a little research on what your reef tank contains, or will contain and consider the right specific gravity for them.

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