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What weighs more, a pound of lead or a pound of feathers? They weigh the same, of course. Now, which occupies more space, a pound of lead or one of feathers? Surely the feathers. If I gave you a definition of density being equal to mass divided by volume which would you say is more dense; lead or feathers?
Not surprisingly, "pure" water is the standard for density. For this phenomenon, at a certain standard temperature and pressure, the density of water is 1.000 (grams per cubic centimeter). Now here comes a key question. What happens when you dissolve something in water in terms of it's density? Well, it depends on the density of that material. Let's say it's a gas that's 'lighter' (i.e. less dense) than water. Then, the density decreases. But most stuff we put in our tanks, salt mix, food- dissolved wastes, water additives, medications are more dense than the water; therefore raising the overall density of the solution.
The concept of density and what makes it up in our captive seas is germinal to understanding that it is not the same as salinity; though they are bandied about as if they were.
The salinity and composition of seawater of reefs varies little around the world. This is not the case in captive systems; and can be a problem. Salts differentially crystallize on the top of tanks, filter sumps, etc. and are lost. Freshwater leaves solution as evaporation. All manner of soluble solids and liquids build-up and increase the density of the captive seawater.
Many aquarists have bought into the notion that specific gravity and salinity are the same. Their systems can get into trouble quickly if they are of small volumes, poorly covered and maintained.
Happily, by following some simple guidelines, specific gravity can be considered a broad equivalent measure of salinity. Keeping this variable homeostatic along with temperature goes a long way in providing an optimum stress-free environment for your livestock.
Salinity is the amount of total salts in a given weight of seawater, generally expressed in parts per thousand (S 0/00). There is, as you could guess, a more formal scientific definition in terms of converted solids in a kilogram of seawater. We don't need to get that involved.
Salinity may be measured by standard methods of boiling a sample down in a laboratory. Chemist-Aquarists "with inquiring minds" can employ titration methods, available in test-kit form, that indirectly through measuring chlorinity (55 percent of salt solids in seawater).
There are refractometric means. Salts bend light that can be gauged in a microscopic-looking tool.
There are other electronic ways, but by and large aquarists don't use any of these; and for good reasons: They cost more money, and simpler specific gravity indicators give good enough service.
Specific gravity (spg) of "fresh" natural or synthetic water can be used to approximate a weight relationship between salts and salinity. The tools that yield values of relative density of seawater are called hydrometers; they come in two basic formats, floating glass and plastic box types.
Glass hydrometers, frequently outfitted with an inner thermometer, range widely in their accuracy. Small, inexpensive models are notoriously bad, while larger (12" plus) ones can be pricey. Let me save you some money and frustration, please. Whatever you pay, get and stay in the habit of using your glass hydrometer in a separate, narrow container; not your system. I could have retired years ago on the money lost in broken glass hydrometers left in tanks and filter sumps. Reading your hydrometer properly is also easier in a static test chamber.
These tube-shaped implements float more or less depending on the density of solution their in. Hydrometers bear calibrated scales on their upper necks. You 'read' the density relative to fresh water (aka specific gravity) as the apparent line across the water's flat surface.
Plastic box type hydrometers are comprised of a small container with an articulating pivot that floats to point at a embossed scale. Get in the habit of rapping the filled box to knock off air bubbles on the pivot, and rinsing the box with freshwater after use to attain/maintain accuracy.
First and foremost know this, there is no one specific gravity/salinity that is perfect for marine systems. I mention this not only so you won't agonize over constant checking and adjustment, but because it's a fact. Though below-surface values are remarkably constant in the wild, in our systems stability is far more important, within a range.
As a general rule, marine systems are best maintained between a specific gravity of 1.020 and 1.025, avoiding changes of more than a thousandth a day.
Fish only systems are frequently kept at artificially low specific gravities (1.017-1.020) commercially for three principal benefits. 1) Cost of salt mix is less; even "natural" water that has to be hauled may be diluted with fresh at a savings. 2) Parasite and microbial levels are reduced; they can't make the osmotic stretch as well as macro-life. 3) Gas solubility is enhanced; increasing capacity, health. The margin of safety in rapid-adjusting fishes to new spg is dependent on several factors, especially their adaptive state. Take care to use a hydrometer to measure the water new charges come in and adjust them over a number of days to system spg (another reason for a quarantine/hospital tank).
Fishes (et alia res.) that originate from the Red Sea, parts of the Indian Ocean where natural specific gravity is high (@ 1.027), Achilles tangs, seahorses, and several types that live in close conjunction with invertebrates (e.g. clown anemone fishes) are exceptions that bear mentioning. These require consistent, higher range densities (1.023-1.025) to fare well.
Systems with invertebrates, algae, and most types of "live rock" similarly appreciate upper range values and stability.
An interesting note on the topic; Walker and Herwig cite several instances correlating rapid reduction of specific gravity with spawning activity. They suggested that lowering spg as a factor in captive breeding along with light and temperature manipulation techniques.
Almost all marine creatures are hypo-osmotic, their bodies are less salty than the surrounding water. Consequently, and in contrast with freshwater, they must consume seawater and eliminate the salts to fight the gradient of freshwater loss. Rapid changes (in particular increases) in salinity/specific gravity challenge their mechanisms for maintaining "salt and solute balance".
Temperature affects are coupled in intensity. All the marine organisms we keep are 'cold-blooded'; their metabolisms are step-linked to water temperature. Both go up or down together.
How much variation in temperature and specific gravity can your specimens tolerate? If they're in good shape otherwise, quite a bit. Rapid and extreme changes are to be avoided however; they all add up to added stress. Shoot for constancy in these measures.
Factors Affecting Specific Gravity Constancy:
Size of the system is a big factor. Small tanks are trouble from several steady-state points of view. Changing water, adding food, a dying organism, temperature fluctuation from power failure, weather, or equipment failure, and more, can all conspire to raise/lower water density quickly. The larger the system, total gallonage, the better.
Evaporation from bubblers, surface area, filtration... all lose freshwater. Don't believe the claims of some of the system and filter suppliers that "water changes", "maintenance" are unnecessary. Simply adding freshwater to replace what has presumably been lost simply to evaporation is a very poor idea. If/when your system is low, it is time to do more than "top off"; a partial water change is called for.
Addition of higher density matter that goes into solution acts to raise spg. Not a pretty picture if all you're doing is shoveling in more food and additives. See above and elsewhere re the virtues of frequent, partial water changes.
Temperature: a typical calibrated hydrometer is adjusted at 60 degrees Fahrenheit. Cooler water is more dense than warmer. A change of ten degrees plus or minus is accompanied by a specific gravity reading change of @ 0.001. Is this a big deal? In the grand scheme of things, no. Just know that if you want to keep your spg at 1.025 at 80 degrees F., set your reading at 1.023 on the hydrometer. Reputable manufacturers of specific gravity measurers will either standardize their hydrometers at tropical temperatures for warm water use, or give a scale, formula for interpretation.
What To Do/Maintenance:
Set up your system to reduce salt and water loss. Gross as it may sound, it is sound practice to scoot the crusty accumulation on the top and sides of your set-up back into the water. This helps to retain consistency in the make-up and spg. The several types of salts that make up synthetic mixes "leave" solution at different "rates" and result in imbalances, especially in small volumes.
When you do your "religious" water changes, take care to adjust the specific gravity of the replacement water to that which you're shooting for. Be aware that even the best salt-mixes dissolve non-instantaneously. Hobbyists in the know incorporate a dedicated canister (with lid), replete with water-mixing pump/airstone, or power-head, heater and thermometer for pre-mixing fabricated synthetic. Great idea.
Specific gravity, which most aquarists use to gauge salinity is not the same as measuring the amount of salts in your system.
Temperature change, adding food et al., evaporation, salt creep and more all contribute to shifts in consistency, density and salinity of your system.
As the cereal ad implores, "What's a mother to do?" Not much really. Set your aquarium up to lose a minimum of water, salts, be chary of overfeeding, adding anything to the water, and do regular partial water changes with water of proper temperature and spg. Oh, and don't worry.
The worrying will do you more harm than any small, gradual change in temperature or spg in your system.
Burgess, Warren E. 1976. Salts from the seven seas (on specific gravity). TFH 12/76.
Walker, Stephen D. & Nelson Herwig. 1976. Salinity and spawning. Marine Aquarist 7(2)/76.