Why is the recommended maximum Salinity lowered, than actually could/(should?) be?

[C.J.]

I live near the Mediterranean sea and continuously record a 1.029-1.03 water salinity, which is much higher than what marine aquarists label as maximum for tanks (generally 1.023-1.026).

I took measurement from the Indian ocean (Maldives) and Atlantic Ocean (Canaries) as well, and both of them were as on the higher end too, close to 1.028-9.

I snorkeled and dived in all sites and as you can expect, I found a large variety of type of marine life that aquarists keep as priced additions to tanks, from fish, shrimps, anemones, crabs, fanworm, etc. to snails and especially in the warmer climates an abundance of corals..

So, Im confused why marine aquarium experts define very low maximum salinity levels compared to what's in the wild?

Eg. just 2-3 meters from the Spanish shorelines of the Mediterranean (at just 1m depth) I've seen peppermint shipms and bumblebee shrimp thriving in the 1.029/30 water. Hovewer, any site will range their keeping between 1.023/24-1.025/26. And lets not forget, the Spanish coastline is one of the least saltiest of the Mediterranean, yet not the only one with a rich wildlife.

To stay at shrimps and crabs, I've seen countless boxer, cleaner shrimp and also spider crabs thriving along the coastline.

So, to all the experts who define a maximum aquarium salinity of 0.25/26.. my question is, why?

 

[Timfish]

Because people just come up with numbers. Here's a paper showing for zooxanthellae the optimum salinity is 38 PSU.

https://www.int-res.com/articles/meps/181/m181p309.pdf

 

[Randy Holmes-Farley Verified Community Expert]

I live near the Mediterranean sea and continuously record a 1.029-1.03 water salinity, which is much higher than what marine aquarists label as maximum for tanks (generally 1.023-1.026).

I don't agree with that premise. I've never said that a sg of 1.026 should be a maximum.

My recommendation has always been that a good target range is 34-36 ppt. The sg of 36 ppt seawater is 1.0271.

The world ocean average is 35 ppt (hence the middle of my target) with a range around that, and a few places significantly higher.

Reef aquaria are quite forgiving about salinity, and there's little evidence that any particular value is "best".

My recommendations also say:

Salinity

There are a variety of different ways to measure and report salinity, including conductivity probes, refractometers, and hydrometers. They typically report values for specific gravity (which has no units) or salinity (in units of ppt or parts per thousand, roughly corresponding to the number of grams of dry salt in 1 kg of the water), although conductivity (in units of mS/cm, milliSiemens per centimeter) is sometimes used.

Somewhat surprisingly, aquarists do not always use units that naturally follow from their measurement technique (specific gravity for hydrometers, refractive index for refractometers, and conductivity for conductivity probes) but rather use the units interchangeably.

For reference, natural ocean water has an average salinity of about 35 ppt, corresponding to a specific gravity of about 1.0264 and a conductivity of 53 mS/cm. It often ranges from 34-36 ppt over reefs, but can be higher or lower locally for various reasons such as land run off of fresh water, or evaporation from a lagoon.

As far as I know, there is little real evidence that keeping a coral reef aquarium at anything other than natural levels is preferable. It appears to be common practice to keep marine fish, and in many cases reef aquaria, at somewhat lower than natural salinity levels. This practice stems, at least in part, from the belief that fish are less stressed at reduced salinity. I have no idea if that is true or not, but I've not seen evidence that it is true. Substantial misunderstandings have also arisen in the past among aquarists as to how specific gravity really relates to salinity and density, especially considering temperature effects. For example, the density of seawater is less than the specific gravity, and measurements with glass hydrometers may require temperature correction, but newer devices do not need the aquarist to make corrections. Consequently, older salinity or specific gravity recommendations may not actually be referring to the same measurements that aquarists make today, even if the recommended numbers have been handed down

My recommendation is to maintain salinity at a natural level. If the organisms in the aquarium are from brackish environments with lower salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make good sense. Otherwise, I suggest targeting a target salinity of 35 ppt (specific gravity = 1.0264; conductivity = 53 mS/cm).

 

[Subsea]

I run 1.03 specific gravity on Caribbean theme tanks that are heavy on filter feeders & sponges.