1.1.1 Background

* U.S. EPA requirements for water quality from municipal sources are insufficiently pure for reef tank usage. For instance, the EPA standard for Nitrate (as NO3-N) is 10.0 mg/l, over twice the recommended maximum level. Extremely toxic (to inverts) heavy metals such as copper are allowed at levels as high as 1 mg/l.  Most public water supplies have contaminants well below the EPA levels and some reef tanks have done fine on some public supplies.

In general, however, it is recommended that some form of post processing be performed on public water before it is introduced into the reef tank.

Although some people have access to distilled, de-ionized or reverse osmosis water from public sources, most will use a home sized system to produce their tank water. The two most common systems used are de-ionization resins, and reverse osmosis membranes.
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1.1.2 DI filters

De-ionization (DI) units come in two basic varieties: mixed bed and separate bed. Two chambers are used in separate bed units, one for anion resins (to filter negatively charged ions), the other for cation resins (to filter positively charged ions). Mixed bed units use a single chamber with a mix of anion and cation resins.

DI units are 100% water efficient with no waste water. They are typically rated in terms of grains of capacity (a grain is 0.065 grams). Once the capacity of the unit is reached it either needs to be replaced or recharged (using strong acids and bases).

Recharging is normally only an option for separate bed units. A quick check of the local water quality reports (normally available free from the water supply company) will reveal the water purification capacity of a given DI unit. For example, if a unit rated at 1000 grains is purchased and the local water supply has a hardness of 123 mg/l (Missouri River, USA), then the unit capacity is (1000*0.065)/0.123 = 528 liters = 139.5 gallons of purified water.

Water production rates for DI units varies, but is typically around 10-15 gallons/hour.   Note that some contaminants captured by a DI unit may "break through" long before the unit indicates its capacity has been reached. Silica is a classic example.   What happens is that silica is loosely bound to the resins initially, but is replaced by stronger binding materials like carbonates as the resins become exhausted.   The use of two DI units in tandem, as mentioned elsewhere in this FAQ, helps to eliminate this problem.
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1.1.3 RO Filters

Reverse osmosis (RO) units are normally based upon one of two membrane technologies: cellulose triacetate (CTA) and thin film composite (TFC). CTA based systems are typically cheaper and do not filter as well (90-95% rejection rates). TFC based systems cost more but have higher pollutant rejection rates (95%-98%).

CTA membranes break down over time due to bacterial attack whereas TFC membranes are more or less impervious to this. CTA units are not recommended for reef tank purposes. TFC membranes are very sensitive chemically to the chlorine found in most water supplies.   It is therefore very important to regularly replace the carbon block pre-filter associated with all better-grade TFC systems. TFC membranes are damaged by chlorine so a properly functioning GAC prefilter is mandatory.

RO filters work by forcing water under pressure against the membrane. The membrane allows the small water molecules to pass through while rejecting most of the larger contaminants.  RO units waste a lot of water. The membrane usually has 4-6 times as much water passing by it as it allows though. Unfortunately, the more water wasted, the better the membrane usually is at rejecting pollutants. Also, higher waste water flows are usually associated with longer membrane life. What this means in practice is that 300 gallons of total water may be required to produce 50 gallons of purified water.

Like any filter, RO membranes will eventually clog and need to be replaced. Replacement membranes cost around $50-$100. Prefilters are often placed in front of the membrane to help lengthen the lifetime. These filters commonly consist of a micron sediment filter and a carbon block filter. The micron filter removes large particles and the carbon filter removes chlorine, large organic molecules and some heavy metals. Of course, the use of prefilters makes initial unit cost more expensive but they should pay for themselves in longer membrane life.

RO units are rated in terms of gallons per day of output with 10-50 gallon/day units typically available. Note that the waste water produced by a RO unit is fine for hard water loving freshwater fish such as Rift Lake cichlids. Some route the reject water to the family garden.

The Spectapure brand of RO units has a good reputation.
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1.1.4 Further Comments About Water

The ultimate in home water purification comes from combining the two technologies and processing the water from an RO unit though a DI unit. If a very high grade DI unit is used, water equivalent to triple distillation purification levels can be achieved. Since the water entering the DI unit can be 50 times purer than tapwater, the DI unit can process 50 times as much before the resins are exhausted. This significantly reduces the replacement or recharging cost of the DI unit. Using two DI units in tandem, moving the 2nd in as a replacement for an exhausted 1st unit, and replacing the 2nd unit with a new unit will insure that no undesirable elements "break through" the exhausted 1st unit and enter your supply.

If only one filter can be afforded, and waste water is not a concern, then it is recommended that a TFC RO unit with pre-filters be purchased.  If waste water is a concern, or if only a small quantity of make-up water will be required (say, for a single 20 gallon tank), then a DI unit would be the preferred choice.

City water is unstable. Many cities modify their treatment process several times a year, dramatically changing its suitability for reef usage. For instance, Portland has great reef water - most, but not all, of the year.
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1.2 Additives

Calcium (Ca) - required addition. A range of 400-450 ppm Ca++ (10-11 mM) is recommended. The preferred method is the usage of Kalkwasser (Limewater) for all evaporation make-up water. The use of Calcium Chloride (CaCl2) is known to cause problems with alkalinity (provable by balancing the relevant chemical reactions occurring in the tank when CaCl2 is added) and contributes to a shift in the ionic balance of the water which must be corrected via water changes. Still, CaCl2 is occasionally useful to repair serious Ca++ deficits.

Calcium Carbonate - The use of Calcium Carbonate reactors is growing in popularity as a replacement, or sometimes in addition to, Kalkwasser usage. Such reactors use CO2 recirculated through a bed of calcium carbonate (typically crushed coral like Geo-Marine) to reduce the pH and dissolve the calcium carbonate. A small fraction of the recirculated water is allowed to re-enter the aquarium and is replaced in the reactor with fresh tank water on a continuous basis. These systems are considerably more mechanically complex than Kalkwasser systems, often involving CO2 tanks, electrical valves, pH controllers, bubble counters, circulation pumps and related equipment. Once setup and tuned for the calcium demand in a tank, they can often be left alone for months. This low maintenance requirement is a primary benefit of the system, combined with the potential to inject more calcium into the system than kalkwasser alone could do due to kalkwasser's low solubility in make-up water. Note that calcium reactors may add residual CO2 to the system which can fuel algae growth. This extra dissolved CO2 may be purged by either enhanced gas exchange or by  adding a small amount of kalkwasser to scavenge the CO2. Newer reactors often have a second carbonate stage to process and utilize this extra CO2. The efficiency of such a second stage is unknown to this author.

Two Part Calcium Solutions (CaCl2) - A third approach to calcium addition is the use of the newly available two-part ionically balanced solutions. These solutions use CaCl2 as a calcium source, but combine that material via the 2nd part solution with complex ion formulas that negate the normal problems associated with CaCl2 usage. These two-part solutions are relatively expensive for large aquariums but may be cost-effective for smaller tanks relative to the capital required for a calcium carbonate reactor. Others simply find their usage more convenient than the other alternatives. These additives will slowly raise the salinity of the water as a side effect. As always, monitor and correct your salinity as necessary.

Chelated calcium - The efficacy of chelated calcium products available for reef aquaria is questionable. To the best of our knowledge, there exists no scientific evidence indicating that chelated calcium is especially available to corals and other CaCO3 depositing invertebrates. Nothing is known about the uptake of chelated calcium products by coral. And most importantly, there exists no evidence showing that chelated calcium products support stony coral growth rates in excess of, or even *comparable to* growth rates documented in aquaria where calcium is supplied as aqueous Ca(OH)2 [kalkwasser.]

Chelated calcium products also interfere with the ability to measure actual calcium levels in the aquarium. In particular, chelated calcium cannot be measured by any kit which uses EDTA titration, including the highly recommended HACH kit.  Until such a time as vendors supplying chelated calcium products make available well conceived, carefully documented uptake and growth studies with their products, or the same experiments are performed and published by third parties, we regard the use of chelated calcium products in the reef aquarium to be experimental at best, especially when kalkwasser and other non-chelated calcium sources are KNOWN to us to support the growth and even reproduction of stony corals in the home aquarium.

Iodine () - SeaChem and Salifert have recently introduced test kits which are finally allowing a view of actual usage in a reef tank. Note that iodine is naturally present in ocean water at relatively low levels (around 0.06 ppm - yes that 60 parts per BILLION).  It is currently considered important for both soft coral growth and hard coral health.  It is removed via skimming, activated carbon usage, and assimilation into biomass. It may also be removed by unidentified processes like precipitation.

Strontium (Sr) - used rapidly by most hard corals (weekly additions usually performed). Test kits are becoming available but the accuracy of current kits is still questionable. Natural ocean water levels of strontium is around 8 ppm.

Buffers - increase alkalinity and control pH. Desired range is 2.5-3.5 meq/L (7-10 dKH) alkalinity. Alkalinity can be raised by the addition of one of many commercial buffer compounds. The addition of kalkwasser (saturated Ca(OH)2 solution - also known as "limewater"), which is often done to maintain calcium levels, will help maintain the alkalinity level. SeaChem's Marine Buffer, Reef Builder and Kent's Superbuffer dKH are popular. The Coralife and Thiel buffer products have had less favorable reviews.

Iron (Fe) - Used by alga. Add this if you want good macroalgae growth. Be sure that macroalgae growth is favored or else plague levels of hair algae may result.

Copper (Cu) - Used as a medication in fish-only tanks. Copper is highly toxic to invertebrates, even in very small concentrations.  DO NOT USE THIS, IN ANY FORM, EVER, IN A REEF TANK OR ANY TANK WHICH CONTAINS INVERTEBRATES. PERIOD!

Other additives, especially the commercial "secret formula" mixtures, are more controversial. Some people report good results from some of them other people report disaster or no effect.

Experiment cautiously with them if desired. We recommend that all products that refuse to reveal what's in them be used with a great deal of caution.
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1.3 Testable Parameters

Note: parts per million (ppm) and milligrams per liter (mg/l) are virtually identical in seawater and the units are used synonymously in this document.
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1.3.1 Alkalinity

Alkalinity is a measure of the acid buffering capacity of a solution.  That is, it is a measure of the ability of a solution to resist a decrease in pH when acids are added. Since acids are normally produced by the biological action of the reef tank contents, alkalinity in a closed system has a natural tendency to go down. Additives are used to keep it at a proper level.

Correct alkalinity levels allow hard corals and coralline algae to properly secrete new skeletal material. When alkalinity levels drop, the carbonate ions needed are not available and the process slows or stops.

Alkalinity is measured in one of three units: milliequivalents per liter (meq/l), German degrees of hardness (dKH) or parts per million of calcium carbonate (ppm CaCO3). Any of the units may be employed but dKH is most commonly used in the aquarium hobby and meq/l is used exclusively in modern scientific literature. The conversion for the three units is: 1 meq/l = 2.8 dKH = 50 ppm CaCO3  [As an aside, there is an imperial unit of alkalinity and hardness which is 'grains per gallon'. The water softening industry uses this unit. 1 gpg = 17 ppm CaCO3.]

A word of caution about the ppm CaCO3 unit is in order. The 'ppm CaCO3' unit reports the concentration of CaCO3 in pure water that would provide the same buffering capacity as the water sample in question. This does not mean the sample contains that much CaCO3.   In fact, it tells you nothing about how much of the buffering is due to carbonates, it is only a measure of equivalency.

Alkalinity is often confused with carbonate hardness since both participate in acid neutralization and test kits may express both in either of the three units. However, carbonate hardness is technically a measure of only the carbonate species in equilibria whereas alkalinity measures the total acid binding ions present which may include sulfates, hydroxides, borates and others in addition to carbonates. In natural seawater, though, carbonates make up 96% of the alkalinity so equating alkalinity with carbonate hardness isn't too far off. As long as you're using a salt mix which yields an ion mix close to that of Natural Sea Water (NSW) you can also make this assumption. Some salt manufacturers alter the alkalinity component of their mix to increase the percentage of borates to (bi)carbonates in order to maintain a stabler pH in the aquarium. We do not feel this is good, and highly recommend you watch the trade magizines for reports on borates in salt mixes.  (OK, OK, here's a preview... Instant Ocean does NOT have abnormal borates based on initial testing.)

Recommended values for alkalinity vary depending on who's work you read. Natural surface seawater has an alkalinity of about 2.4 meq/l. Following are levels recommended by various authors.

From John Tullock (1991) "The Reef Tank Owner's Manual": page 46 - Alkalinity range should be 3.5 to 5.0 meq/l.  page 94 - Alkalinity reading of 2.5-5.0 meq/l is proper.  page 188- Alkalinity should be about 3.5 meq/l. (In reference to maintaining Tridacna clams.)

From Albert Thiel (1989), in "Small Reef Aquarium Basics" recommends 5.35-6.45 meq/l. This is an artificially high level which may initiate a "snowstorm" of CaCO3 precipitate. Most reef aquarists do not believe in such extreme and unnatural levels and recommend 3.0-3.5 meq/l as a good range instead.

The chemistry of how alkalinity, pH, CO2, carbonate, bicarbonate, and other ions interrelate is fairly complex and is beyond the scope and detail of this document.   Some recommended test kits for alkalinity are the SeaTest kit, the inexpensive Tetra kit and the LaMotte kit. The SeaTest kit measures in division of 0.5 meq/l or, if the amount of solution is doubled, 0.25 meq/l. The SeaTest kit uses titration in which the acid and indicator are included in the same reagent. The LaMotte kit is a little more expensive, though still fairly cheap, and is somewhat more accurate. The unit of   titration is 4 ppm CaCO3 although in practice, one drop from the titration tube may be up to twice this amount making the resolution about 0.15 meq/l. The Lamotte kit has a separate indicator tablet and acid reagent which is a nice feature.
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1.3.2 Calcium

Calcium content is referred to as 'calcium hardness' and is measured either in parts per million of calcium ion (ppm Ca++) or parts per million equivalent calcium carbonate (ppm CaCO3).  Calcium hardness is often confused with alkalinity and carbonate hardness since the 'ppm CaCO3' unit may be used for all three. As with alkalinity, a calcium level expressed as X ppm CaCO3 does not imply that X ppm of calcium carbonate is present in the tank; it merely states that the sample contains an equivalent amount of calcium as if X ppm of CaCO3 were added to pure water. The reading also does not tell you how much carbonate is present.  Calcium hardness test kits are different from alkalinity kits.

Some people have reported difficulties with the LaMotte calcium hardness kit. The Hach 'Total Hardness and Calcium' kit has not had these reports. Both express results in ppm CaCO3. The relationship between CaCO3 and Ca++ is:

1 ppm CaCO3 = 0.4 ppm Ca++

The results from a test kit reading in ppm CaCO3 may be converted to the molar concentration scale by dividing by 100. 

100 ppm CaCO3 = 1 mM Ca++
40 ppm Ca++ = 1 mM Ca++

Calcium levels of natural surface seawater are around 420 ppm Ca++ (10.5 mM). In a well running reef tank you will notice, sometimes dramatic, calcium depletion. Calcium addition in some form is essential. A calcium level above 400 ppm is required and a range of 400-450 ppm Ca++ is recommended. Most reefkeeping books (see bibliography) explain the options for calcium addition.
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1.3.3 pH

The suggested reef tank range is 8.3 to 8.4. The pH should hold its own unless alkalinity is low. If alkalinity is OK but pH is low there is probably a buildup of organic acids or a serious lack of gas exchange resulting in the retention/accumulation of CO2 which lowers pH.

Note that it is perfectly normal for the pH of a tank to swing considerably. There is a daily pH cycle where the pH is lowest just after the end of the dark period and highest sometime before the end of the light period. Having a pH range from 7.9 to 8.4 is not unheard of. Larger swings are probably indicative of low buffer levels or poor gas exchange.
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1.3.4 Nitrate (NO3)

Two units are used to measure nitrates: nitrate (NO3-) and nitrate nitrogen (NO3-N or just N). The ratio is:

1 ppm NO3-N = 4.4 ppm NO3-.

Nitrates themselves may not be a problem but serve as an easily measured indicator of general water quality. Many hard to test for compounds like dissolved organics tend to have levels that correlate well with nitrate levels in typical tanks.

Different authors cite varying upper nitrate values permissible. No higher than 5 ppm NO3- is a good number with less than 0.25 ppm recommended. Unpolluted seawater has nitrate values below detectable levels of hobbyist test kits, so "unmeasurable" is the goal to strive for.

Most test kits measure nitrate-nitrogen. Do not forget to multiply by 4.4 to get the ionic nitrate reading. LaMotte makes a nitrate test kit that will measure down to 0.25 ppm NO3-N.  The Hach kit, which measures down to 0.02 ppm N03-N has basic chemistry problems in saltwater and is no longer recommended.
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1.3.5 Phosphate (PO4)

Phosphates, along with nitrates, are a primary nutrient of algae.  Tanks with "high" levels of phosphates tend to be infested with hair algae. All authors cite zero ppm PO4 as a good goal. An upper level 0.1 ppm is recommended by Tullock (1991) with less than 0.05 ppm given by Thiel (1991).

The use of kalkwasser has been closely tied with reduction in phosphate levels. This may be due to precipitation of the phosphates at the kalkwasser injection site, or, more likely, due to increased export via skimming due to the associated higher system pH.
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1.3.6 Specific Gravity

Short form:

Specific Gravity is temperature dependant. See the next table for a quick lookup of the recommended hydrometer readings. They are based upon our recommended S.G. of 1.025 at 60 degrees F.

Degrees F. Hydrometer reading.

50 1.0255
55 1.0252
60 1.0250
65 1.0246
70 1.0240
75 1.0233
80 1.0226
85 1.0218 (rather hot for most tanks)
90 1.0210 (very hot for most tanks)

In more detail:

1.025 recommended for reef tanks. Note that virtually all hydrometers are calibrated for measurements at a temperature of 60 F. Included below is a short table of temperature adjustments. Add the value shown to your hydrometer reading to get an accurate reading.

Degrees F. Correction

50 -0.0005
55 -0.0002
60 0.0000
65 0.0004
70 0.0010
75 0.0017
80 0.0024
85 0.0032
90 0.0040

For example: If the hydrometer reads 1.0235 at 80F, the actual Specific Gravity is 1.0235 + 0.0024 = 1.0259

Note: If your tank is between 75F and 80F, this means you should try and keep your Specific Gravity around 1.0230 to 1.0235.

For all practical purposes, the scale is linear between data points, so you can simply extrapolate between table entries. For instance, 78F is 3/5 the distance between 75F and 80F; the difference in corrections is 0.0024-0.0017 = 0.0007. 3/5th of 0.0007 is 0.0004. Add the offset 0.0004 to the base value for 75F of 0.0017 and you get a correction value for 78F of 0.0021.

It is fairly common in the literature to see references to salinity in terms of Parts Per Thousand (PPT). For salinities in the range we are interested in, the conversion formulas are:

Salinity = 1.1 + 1300 * (Temperature corrected Specific Gravity - 0.999)
Temperature corrected Specific Gravity = ((Salinity - 1.1) / 1300) + 0.999;

Here is a short table of some common values:

Salinity Specific Gravity

20 PPT 1.0135
25 PPT 1.0174
30 PPT 1.0212
35 PPT 1.0251 * Typical Ocean Value *
40 PPT 1.0289
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1.4 Water Changes