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Water Changes in the Reef Aquarium

By Troy Brightbill

 Water Changes in the Reef Aquarium

Recent discussions on the effectiveness and actual need of frequent water changes in the reef aquarium led me on a quest to quantify the actual results achieved when conducting these water changes.  Many reef aquarists perform 10-25% nominal water volume changes on a weekly to bi-monthly basis.  Some do so in an attempt to correct elevated nutrient levels, such as nitrate and phosphate, or to reduce toxin levels.  Others conduct these frequent water changes as a means of replenishing calcium in their systems, or to replenish lost trace elements.  Even more often it seems, you'll hear someone say that a water change will cure whatever problems there may be in a reef aquarium.  So, I drank a big pot of coffee, fired up the calculator and went to work on it. 

The water volumes used in these calculations are estimated nominal water volumes .   In other words, the actual volume of water in the tank, not including space taken up by live rock, sand or animals, is used for the calculations.  I chose to use two common tank sizes as examples.  Comparisons between 10 gallon (37.9L) and 55 gallon (208.2L) systems are made in this document.  I used an estimated 8 gallons (30.3L) nominal volume for the 10 gallon tank, and an estimated 40 gallons (151.4L) nominal volume for the 55 gallon tank.  The actual nominal water volume varies greatly from tank to tank - the best way to find yours is by measuring how much saltwater was added to your tank after placing live rock and sand in the tank.

Correcting Excess Nutrient Levels

My line of reasoning is that a properly run tank (big or small) won't need water changes to correct problems.  But in the event that they do, here is some actual data on water changes.

To correct a nitrate problem:  Assume you have 20mg/L nitrate, and seek a target level of 5mg/L.  Assume that there is no further nutrient buildup during the period of water changes ( which is pretty unlikely - since nutrient import has been greater than nutrient export thus far, it would take even longer unless corrected with additional filtration ).  Each change number represents one water change, done at one week intervals.  Bi-monthly water changes will take twice as long to correct such a problem.  Note - these calculations apply to all tanks, no matter what size.  If you perform 25% changes on a 10 gallon tank, it takes just as long as doing 25% changes on a 55 gallon tank to reach the same target level.

Figure 1 consists of calculations showing how many water changes are necessary at given percentages of the nominal water volume to reduce an elevated nitrate level. 

Using 25% changes, it would take 5 water changes and 10 gallons of water to lower nitrate from 20mg/L to 5mg/L in 8 gallons of saltwater contained in a 10 gallon reef tank, assuming no additional nitrate buildup in the system.  Along the same lines, it would take 5 changes for a 55 gallon tank containing 40 gallons of saltwater, requiring a total of 50 gallons of new saltwater. 

It would take 14 water changes and 11.2 gallons of saltwater to lower nitrate from 20 mg/L to 5 mg/L using 10% water changes, again, assuming no additional nitrate buildup in the 10 gallon system.  It would also take 14 changes in the previously mentioned 55 gallon tank, requiring 56 gallons of new saltwater.  By using the graph in Figure 2 , one can find their current nitrate level and determine how many water changes it would take to lower the nitrate to a target level.

Another nutrient that can build up in reef aquariums is phosphate.  To lower phosphate  from 2.0 mg/L to 0.2mg/L using 25% nominal volume water changes would take 8 changes and 16 gallons of saltwater for the 10 gallon system (for the 55 gallon - 32 gallons new saltwater) .  Using 10% water changes, it would take 22 changes- 17.6 gallons of new saltwater for the 10 gallon system, and 88 gallons for the 55 gallon system!  The same applies for reducing toxins in your tank - activated carbon would be much more effective at reducing toxins such as terepenes which may build up in your reef aquarium.  With Figure 3 , one can find their current phosphate level and find how many water changes are necessary to reach a target level.

Replenishing Iodine

It has been argued that all additional supplements for reef aquaria may be replaced by frequent, small water changes.  Take iodine for instance.  Iodine has been shown to be a vital trace element in natural saltwater, found at a concentration of 60 parts per billion (0.06mg/L).  Some iodine may be added to the system from feeding, but I am far from knowing the effectiveness of food as a means of supplying iodine.  Most aquarists who test for iodine find that they have to add supplemental iodine in the form of potassium iodide or lugol's solutions - this includes aquarists who perform weekly water changes and those who don't.   Potassium iodide is a fairly inexpensive additive that should not be overlooked.

Replenishing Calcium With Water Changes

All reef aquariums will have some calcium demand.  Studies show that depending upon the species present in a given area, a coral reef can create up to 20 kg/m 2   of calcium carbonate per year.  To quantify this, let's first apply it to the lower levels of calcification, 4kg/m 2   per year (more data on this can be read in Craig Bingman's March 1998 column in Aquarium Frontiers ).  Next, a common tank size - a 55 gallon all glass tank, with a footprint of approximately 4' by 1'.  This gives a surface area of 4 ft 2 , or 0.37m 2 .   This would equate to a 1.49kg demand of calcium per year using the previously calcification rate.  As this is an ideal situation, we will ignore the possibility of calcification outside the main reef tank, such as on hardware.  1.49kg = 1490g per year.  Dividing this by 365 days in a year gives an average daily production of 4.0g calcium carbonate.

From this number, we must now find the mass of actual ionic calcium used on a daily basis.   One mole of CaCO 3 weighs  100.1g.  Dividing the molar mass of Ca (40.1g) by the molar mass of CaCO 3 yields a figure of 40% calcium ion by mass per calcium carbonate molecule.  So to find the actual amount of calcium used per day in this ideal situation, multiply 4.0g by 0.4 to get a total of 1.6 grams of calcium ion used per day in this reef aquarium.  In Figure 4 , this demand is approximated to be 10.6mg/L calcium per day, and is then graphed accordingly, with water changes done on every seventh day.  This theoretical graph shows what one could expect when using only water changes to replenish calcium levels.

Now, assume that the artificial salt mix used gave an initial calcium level of 400mg/L.   The nominal volume of a 55 gallon aquarium is estimated to be 40 gallons for this exercise.  40 gallons = 151.4L nominal water volume.  400mg/L of calcium in 151.4L saltwater gives a total of 60.6g of calcium ion present in the system initially.   Each week, the system consumes 11.2 grams of calcium.  So after one week, the total calcium ion present would be 60.6g - 11.2g = 49.4g calcium, resulting in a new calcium level of 326.3mg/L.  Assume that all water changes are done with properly mixed saltwater of the same salinity  maintained in the reef aquarium.

A 10% water change would replenish 4 gallons - 15.1L - of original saltwater with saltwater containing 400mg/L of calcium.  So the amount of original saltwater left over would be 136.3L (at 326.3mg/L, 44.5g Ca total), and the new saltwater would be 15.1L (at 400mg/L, 6.1g total).  After the 10% water change, 50.6g of calcium would be present in the system. 

This equates to a calcium level of 334.2 mg/L.  

Compare this to the initial calcium level of 400mg/L!  The calcium level dropped 73.7mg/L in one week, yet the calcium level was only raised 7.9mg/L with a 10% water change.  This is after just one week, yet the trend will continue to increase as time goes on, given that water changes are the only source of calcium.  Yes folks, even in a small tank with just soft corals and coralline algae, you will have continually decreasing calcium levels when relying solely on water changes for calcium.

For weekly 25% nominal water volume changes, we would remove 10 gallons - 37.9L - of saltwater per week.  Using the previous calculations, in one week the calcium level would once again drop to a level of 326.3mg/L, with a total of 49.4g calcium present in the system.  Changing 37.9L leaves 113.5L original saltwater (326.3mg/L, 37.0g Ca total), and adds the 37.9L of saltwater at 400mg/L (15.4g Ca total).

This adds up to 52.4g total calcium in the system, and a calcium level of 346.1mg/L.

  A 53.9mg/L drop in the calcium level after just one week and one 25% water change.   Once again the water changes fall short of replacing all the calcium used in the system in the last week, and this trend will continue as with the 10% changes.  In short, water changes alone will never maintain the initial calcium level. 

There are some very simple solutions for this problem, fortunately.  A preferred method of calcium supplementation comes in the form of calcium hydroxide, known as "kalkwasser"to reef aquarists.  Calcium hydroxide has a chemical formula of Ca(OH) 2 , in other words, it supplies one Ca +2 ion for every two OH - ions.  Calcium hydroxide can be mixed at approximately 2 teaspoons [5.7g Ca(OH) 2 ] per gallon (or 1.5g/L) of filtered freshwater to attain approximately 800mg/L calcium solution to be dripped slowly into the reef aquarium.  A beneficial side effect of using calcium hydroxide is that the OH - (hydroxide) ions maintain the pH of the aquarium by neutralizing acids present in the aquarium, thereby helping to maintain (but not build) the alkalinity in the aquarium.  One could use pickling lime, which is also calcium hydroxide.   Unfortunately, pickling lime tends to have visible amounts of impurities in it.

Assume the previously mentioned 55 gallon reef aquarium has two gallons of water evaporated per day, which is replaced by dripping in a calcium hydroxide solution.  Two gallons = 7.6L of calcium hydroxide, which at 800mg/L calcium gives a total of 6.1g of calcium to be added to the system.  This more than accounts for the minimal calcification rates described above for this system.  Even with a lower evaporation rate of one gallon per day, 3.1g of calcium is replenished each day - again, exceeding the calcium demand for this system.  In light of that fact, one can either dose a weaker mix of calcium hydroxide, or allow for more calcium-dependant invertebrates to be added to the system.

Consider the following:  At most retail reef-related stores, one can purchase 450g of kalkwasser mix for about $10.  At a rate of even 5-10 grams used per day for the 55 gallon system described here, this mix would last for 45-90 days.  This is a very minute cost of $0.11 to $0.22 per day to maintain calcium levels.  Since you already paid for your hardware and livestock, why skimp now - especially when it is so inexpensive?

Non-ideal aquariums will lose additional amounts of calcium in plumbing, powerheads, in the protein skimmer, or even from salt creep.  Many reef aquarists keep invertebrates with higher calcium demands, and for these situations, several other methods may be implemented to maintain calcium and alkalinity.  Calcium carbonate dissolution in sand beds is believed to occur, but in very small amounts which are difficult to measure.  I personally would not rely on this alone to supply calcium to the system.

Correcting Magnesium Levels

The amount of magnesium precipitation that occurs in reef aquariums is very small compared to calcification rates.  However, without periodic monitoring, the level can drop.  In some cases, people have discovered they have drastically low magnesium levels, possibly a result of bad artificial saltwater mix.  In light of this, Dallas Warren asked me to include some data on correcting magnesium deficiencies.

An initial concentration of 600mg/L was chosen.  In natural saltwater, magnesium is found at roughly three times the concentration of calcium.  A target level of 1200mg/L has been selected.  Using the graph in Figure 5 , one can find how many weeks it takes to reach 1200mg/L using water changes, for any tank size.  Using 25% or 50% changes, you will see that the graph begins to plateau as the magnesium level nears 1200mg/L.  The same occurs as the levels approach 1200mg/L using 10% water changes.  Yet with 25 weeks of performing a 10% change per week, the magnesium level is still not up to 1200mg/L.  It would take 16 water changes at 10% water volume to reach even 1100mg/L.  Using 25% changes, it would take 6 changes to reach 1100mg/L, and about 18 changes to finally reach 1200mg/L.  Using 50% water changes - which by most accounts are stressful on reef tank inhabitants - would take 3 changes to clear 1100mg/L, and about 12 changes to reach 1200mg/L.  Notice the large number of changes, for all 3 percentages shown here, required just to go from 1100mg/L to 1200mg/L.  Notice also that water changes are by no means fast nor inexpensive when used to correct such ionic imbalances.

The most common method used by reef aquarists to replenish magnesium is the use of... drum roll, please.... Epsom salts.  Epsom salt is simply hydrous magnesium sulfate, with a molecular formula of MgSO 4 -7H 2 O.  Magnesium makes up 10.5% of Epsom salts by mass.  To replenish 600mg/L in the 55 gallon tank used in this example, 90.8 grams of magnesium is required.  This requires 898.1g of Epsom salts.  I would recommend dividing this amount up to be dosed over a period of atleast one week, if not more.   A 25% to 50% initial water change will reduce the amount required significantly, but after the first water change, each subsequent water change becomes increasingly less effective. 

Hydrous magnesium sulfate will increase the sulfate concentration in the saltwater along with the magnesium concentration.  Unfortunately, most aquarists don't have the means to test their sulfate levels.  Again, an initial water change will help alleviate this problem somewhat.  Another option would be Magnesium Chloride, but it is not as easy to find, and could eventually raise Cl - levels.  More on dosing Epsom salts can be read at Dallas Warren's website (look in the DIY section).

Trace Elements

Recent works have shown that the artificial saltwater mixes on the market today contain higher trace element levels than those found in natural saltwater.  Protein skimming and use of carbon are suspected to remove trace elements to some extent, but there is currently no method of measuring the level of trace elements readily available to reef aquarists.  One would expect the reef animals to make use of these trace elements as well.  Therefore, some method of replenishing these levels must be implemented in our reef aquariums. 

Considering that artificial saltwater mix contains elevated levels of trace elements (Atkinson, data presented at WMC 1998), one can rely solely on water changes to replenish them because the initial levels from the salt mix are higher than the levels found in natural saltwater.   While I do not have the actual data on hand (I'm trying to track it down at the moment), I believe that even a single 10% water change in a month can account for any loss of trace elements in the system.  Many have proposed dosing trace element supplements at as little as one half to one tenth of the prescribed dosage to maintain trace element levels.  In my opinion, trace element replenishment is not a major issue when considering whether or not to conduct frequent water changes.  For those truly concerned, a $10-20 bottle of trace elements supplement will last you a very long time, costing even less per day than the kalkwasser.


The intent of this article is not to scare people out of conducting frequent water changes, nor to single out those who propose conducting frequent water changes.  By frequent, I mean 2-4 water changes per month.  The intent of this article was to debunk the common myths as to why people need to conduct frequent water changes of 10-25% nominal water volume.  Through basic chemical equations, I have attempted to show how long it actually takes to correct elevated nutrient levels in our reef aquariums.  Water changes on the order of 10% to 25% nominal volume, when carried out on a weekly basis, have been shown to be a very slow method for lowering nutrient levels.  A larger initial change of 50% nominal volume will be far more effective, but this usually causes a great deal of stress for the aquarium inhabitants.  I have also attempted to show how water changes alone will not compensate for calcium consumption.  It is a basic mathematical relationship - since the salt mix only supplies these chemicals at levels near natural saltwater levels, it will not make up for uptake of these chemicals when partial water volume changes are performed.  The only true way to have water changes alone maintain the saltwater chemistry is to have a truely open system, such as at Wakiki Aquarium.

Frequent water changes, when performed correctly, will not cause any noticeable harm to your aquarium.  Water changes are required to correct Cl - ionic imbalances, usually resulting from the frequent use of calcium chloride (CaCl 2 ), as nothing will physically remove Cl - alone from the water without ripping out every other anion.  I feel that water changes are most beneficial when used to remove particulate wastes from the substrate of the aquarium, when cleaning plumbing, and when cleaning the protein skimmer.   I prefer to do a monthly water change to perform these tasks. 


Weekly water changes will not maintain calcium levels in a reef aquarium.  Some form of additional input, such as calcium hydroxide, is needed to maintain the calcium level.  It can take months to correct nutrient levels with water changes alone.   This isn't anecdotal evidence, this is clearly displayed in the previous calculations.  I feel strongly that a properly run reef aquarium - utilizing protein skimming, calcium hydroxide dosing, dosing of potassium iodide and other essential trace elements, and a monthly cleaning requiring about a 10% water change - will thrive.  Other than these simple requirements, there are numerous methods people can use. 

It is better to maintain the levels discussed here, rather than have them swing up and down on a weekly basis.  This is a common effect resulting from frequent water changes.  In the preceding text I have questioned the actual need for frequent water changes, and I have attempted to dispel many of the myths about why people need to perform them, as well as myths about what they accomplish. 

For further data and graphs (Figures 1-5), see t_brightbill_wcdata.html .

Created by liquid
Last modified 2006-11-25 19:06