Specific gravity is the ratio of the weight of a given volume of any substance compared to the weight of an equal volume of distilled water at a defined temperature. By definition, then, pure water has a specific gravity of 1. This is a useful measurement for assessing the proper concentration of salts in seawater. As the concentration of dissolved solids heavier than water increases, the specific gravity increases. It is possible to have aquarium water with a specific gravity less than 1.0, if dissolved solids have a weight less than water, as in soft water rich with tannins or other organic acids.

What is a buffer?

A buffer is any agent or mixture that has an acid and base in equilibrium:

H+ + B- <——> HB

As before the equilibrium can be expressed as a K, where K= (H+)(B-)/(HB). At equivalence, the pH of this solution will be the same as the pK, which by analogy with pH, is the negative log of K. A buffer at the same pH as its pK has maximum ability to resist pH change in either direction. A buffer at this pH has its acidity equal to its alkalinity and offers maximum protection. Sea water is the most familiar buffered water: it has several buffer systems, including carbonic acid-bicarbonate, bicarbonate-carbonate-borate, and various ion-ion interactive versions of these. Commercial freshwater buffers are mixtures of monobasic and dibasic phosphates. Phosphates have low solubility so that any liquid versions of such buffers are not good buys. Phosphates, however, are excellent freshwater buffers: they are highly efficient, stable, and non-toxic. Their main drawbacks are poor solubility in hard water and they promote algae growth and proliferation (eutrophic). Sodium bicarbonate, also called acid carbonate, hydrogen carbonate, hydrocarbonate, or baking soda, is also frequently used as a freshwater buffer. Bicarbonate has a serious drawback in freshwater, however, arising from the low dissociation of carbon dioxide in water. Bicarbonate added to water creates the equilibrium:

2NaHCO₃<——>Na₂CO₃ + H₂O + CO₂.

Since CO₂ does not readily dissociate or ionize to carbonic acid, CO₂ is loss to the air, the net effect being that bicarbonate is gradually converted to carbonate unless an acid component is supplied. This is sometimes done by injecting CO₂ in response to pH increase. The main drawback to this approach is dependence on a high maintenance electronic system which can be prone to malfunction, usually electrode-malfunction. Soft acid waters sometimes use organic acids to buffer the water. Peat, wood, and root are abundant sources of such acids, particularly tannic and gallic acids. Such acids, however, tend to discolor and haze the water. A more serious danger is the possible oxidation of these acids to phenols, which are toxic. Several organic buffers that do not have these drawbacks are available.

Is the suggestion to remove the wet-dry from my reef system to lower nitrate concentration sound advice?

Nitrate is the final oxidation product of ammonia and nitrogenous waste arising in the aquarium. Ammonia is released into the water by fish, invertebrates, and other living creatures. Nitrogenous chemicals such as proteins, peptides, amino acids, nucleosides, purines, pyrimidines, etc. are likewise released into the water by the living creatures in it. These nitrogenous chemicals are ultimately metabolized by heterotrophic bacteria to ammonia.

Ammonia is then converted to nitrite, then nitrate by aerobic nitrifying bacteria. Aerobic nitrifying bacteria are omnipresent on all exposed surfaces in the aquarium. Anaerobic denitrifying bacteria are present wherever there is oxygen poor surface. In the reef tank, this anaerobic surface is present primarily inside the porous structure of the “live rock.”
At first glance, it may seem to make sense to decrease nitrate formation by removing some nitrification capacity. And, in fact, if you do it, it may actually seem to work, provided you leave enough nitrifying surface to take care of the ammonia directly released by fish and invertebrates. The problem is that the organic nitrogenous compounds are no longer efficiently converted to ammonia and thus accumulate more rapidly. Their removal becomes much more dependent on skimming and chemical filtration. Since less ammonia is being formed and denitrification is constant, nitrate concentration either drops or remains constant or increases at a reduced rate, depending on the biology of the specific aquarium. Unfortunately, the concentration of nitrogenous organic matter (which is not measured by test kits and some of which is more toxic than nitrate ) will increase and the nitrification of ammonia, while it may remain adequate, will decrease and may reach a critical capacity that is inadequate to handle sudden surges of ammonia There are no magic reef bacteria that bypass nitrification and directly remove ammonia and other nitrogenous compounds. No matter how much we may wish it, the reef aquarium is not the open ocean.
The only alternate route for ammonia and nitrogenous waste is algae and plants. The more algae (coraline, macro, and others) the better. Algae in an external scrubber is even better, because it allows for harvesting and thus actually removes nitrogen products from the system.
It seems more intelligent not to short circuit nitrification and heterotrophic metabolism of organics by removing filter area, but to increase denitrification instead, so that it can keep up with nitrification. How can this be done? Water changes, of course, are labor intensive, but definitely lower nitrate and organics, and should be a routine of intelligent reef management. Increase the quantity of reef rock to increase denitrification. Use properly porous materials such as porous glass, lava rock, or de*nitrate (Seachem), either as bottom substrate or in a separate filter. Some commercial denitrifying filters are available. Those that require feeding with methanol (wood alcohol) or similar materials are probably best avoided. Algae scrubbing is also an effective way to decrease nitrate concentration as well as the concentration of many nitrogenous chemicals. Years ago, before reef aquaria were popular, I maintained successfully for years until a move was required, a combination reef and fish aquarium with an undergravel filter (heresy today!), no wet-dry, but an efficient algae scrubber, with virtually no nitrate.

How concerned should I be about phosphates? Are they toxic?

With the exception of a few organophosphate insecticides, phosphates are not toxic. Monosodium and disodium phosphates are routinely used as freshwater buffers and pH adjusters. In seawater, phosphates are only slightly soluble and precipitate as detritus. Phosphates are inhibitory to hard corals and for that reason are undesirable in reef aquaria. Phosphate is an essential plant nutrient and tends to promote proliferation of algae, particularly undesirable hair algae, in both freshwater and seawater. Algae proliferation, however, also depends on nitrate and potassium concentrations, as well as nitrogenous organics, carbon dioxide, trace elements, and competing plants and macroalgae.