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What is the Best Specific Gravity for a Reef Tank?

Specific Gravity for a Reef Tank

In a reef tank, you’re dealing with what would be considered a “closed system,” where everything living in it must get along because nothing can escape to the relative obscurity of the open ocean. 

And, if you don’t keep track of the specific gravity, it won’t matter, because nothing will survive for long. 

For a reef tank aquarium, you want your specific gravity to be anywhere between 1.023 and 1.025. That’s the best relative salinity to pure water measurement. Too much or too little will shock the system, which is never a good thing. 

There are plenty of delicate organisms in a reef, and getting the salinity wrong is dangerous for them. 

You can measure the specific gravity of your tank by using a refractometer, hydrometer, or a premium electronic salinity monitor. Either way, you decide to go, it’s important to stay on top of it. For instance, you should always measure it before and after you change the water in the tank. 

What is the Optimal Range of Salinity for a Reef Aquarium?

The optimal range is going to be between 32% and 35% salinity, however, that can change depending on what’s in your tank. For instance, the optimal range approaches 38% salinity if you have coral in there from tropical waters. 

That doesn’t mean that you should always have the salinity percentage maxed at 38%. It’s just an indication of the differences in coral, depending on where they come from and the levels of salinity in their native water. For instance, 35% wouldn’t kill it. 

On the reverse side, you want 32% salinity for corals that came from areas closer inland. So it’s important to know exactly what it is that you are putting in your tank and the kind of salinity that is best for it. Some coral may survive for years at 33% while 38% is far too much. 

Though the number for what is considered “optimal” seems to fluctuate with the type of coral, most aquarists will agree that 35% is the normal number for reef tank salinity. This comes out to about 1.026 sg (specific gravity). Of course, it’s difficult to maintain exactly 35% because of evaporation and other, external factors.

How to Make Salt Water at the Right Salinity?

It sounds like a difficult task. Not necessarily making saltwater, but getting the right level of salinity. In the reality, it’s not so difficult a prospect. 

  • Choose and set up your salt mixing container
  • Add your heater
  • Mix with a powerhead
  • Measure the salinity

Depending on how much water you need, you can either use a five-gallon bucket or all the way up to a 50-gallon Tupperware container or garbage can. Just don’t use anything that has been used for storing or carrying other materials that may end up being toxic to the process. 

You can add a heater in order to speed up the mixing process of the salt and water. Once your salt is mixed into standard, room-temperature water, it needs to remain there for 24 to 48 hours, with the optional heater keeping the water at the right level. Ultimately, you want it to be 70°F to 78°F. 

The power head is also optional, as it just speeds up the mixing time, rather than waiting the full length of 48 hours. Use a refractometer or a hydrometer to measure the salinity. Natural seawater is going to be 35ppt, while your specific gravity measurement should be between 1.023 and 1.025. 

All you have to do from this point is add salt to bring the level up or add fresh water to bring the level back down to the right range. 

How to Maintain the Salinity of a Reef Aquarium?

There are four things that you can do to maintain the salinity levels at their optimum amount.

  • Top off with fresh water
  • Match the salinity of new water to old
  • Match the level of water changes
  • Compensate for salt loss

Water is always evaporating and when it does, you need to top it off with additional, fresh water. You don’t want to add salt because the salt didn’t evaporate with the water, so the salt level increases as the water evaporate. 

Before you change out the dirty water, measure the salinity and make sure that the new water coming in has the exact, same amount. This avoids shocking the system and harming or potentially killing your reef. 

In terms of matching water changes, that simply means to put in the same amount of water you take out. You’ll have to compensate for the loss of salt as well, just like you do with water from evaporation. The problem is, salt clings to things and scales up on the rim of the aquarium, reducing the salinity of the water. 

Which Tools to Use for Measuring Salinity

The primary tools that are used for measuring salinity are refractometers and hydrometers. They both measure the salinity/specific gravity of your reef tank water, however, they each have their method and each is read differently. 

Hydrometers are a bit more old-school in terms of usage for measuring salinity. They’ve often been replaced by refractometers, but that doesn’t mean they are any less efficient at what they do. You simply lower the hydrometer into the tank water until the device is full.

Pull it out and set it down. The swingarm (needle) will slowly rise until it stops at the number that indicates the tank’s specific gravity. 

The refractometer measures light, rather than the density measurement of the hydrometer. Or, rather, it measures the bending of light. The density of the liquid affects how well the light is refracted, so you can get an accurate measurement of the specific gravity. 

The refractometer is supposed to be a more accurate measuring device than the hydrometer, however, it needs light to function, so you want to use it only in a well-lit room. They are also notably more expensive than hydrometers, so cost is a consideration as well. 

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