Water Changes. Benefits and Caveats

One of the main challenges facing any reef aquarium hobbyist is the maintenance of water chemistry conditions within recommended ranges. This maintenance involves the removal or reduction of detrimental compounds (e.g., excess phosphates and nitrates, biological toxins, etc.) along with the replenishment of other beneficial consumed elements (e.g., calcium, magnesium or trace elements).

For the removal of excess nutrients, both organic and inorganic, there are numerous techniques available, for example, skimmers, activated carbon or GFO. For the replenishment of calcium, magnesium and alkalinity, the balling system or calcium reactors are good options, among others. We can say that nowadays the hobbyist has at his disposal an extensive menu of valid and proven options to maintain optimal water quality in his aquarium.

Water changes are one of these techniques, namely rapid export, probably the oldest due to its simplicity , since they do not require any complicated technological process, nor sophisticated equipment in manual mode. In its simplest version, the make-up water should try to maintain the same chemical parameters as the existing water in the tank: ionic concentrations, salinity, alkalinity, pH and temperature.

Benefit of water changes
- The most significant benefit is undoubtedly the removal of nitrate and phosphate in excess from the aquarium water, in a percentage equal to the ratio between the partial volume exchanged and the total net volume of the aquarium, including the sump. The change involves an abrupt decrease in these concentrations and those of other existing compounds not contained in the exchanged saltwater, according to this ratio.

- Removal of biological toxins, waste products of biological activity and harmful compounds that mechanical or chemical filtration (skimmer, fleece, activated carbon) has not previously been able to remove for any reason, e.g., refractory organic substances such as phenols.

- Removal of unintentionally introduced contaminants, e.g., excess trace elements, copper, aluminum and other toxic metals in excess.

- Contribution to the restoration of ionic balance, i.e., the relationship that exists in natural seawater between the concentrations of constituent ions: calcium, magnesium, carbonates, bicarbonate, sulfate, chloride, sodium, strontium or boron, among many others. This is very important when using Balling or unbalanced calcium and alkalinity additives.

- A weekly or monthly routine of water changes keeps you in touch with the aquarium, facilitating the hobbyist's bond of interest and increasing opportunities for observation so that you can anticipate problems before it is too late.

Disadvantages of water changes
As indicated, a water change implies an abrupt reduction in the concentrations of elements in the aquarium that are not part of the natural seawater, for example, nitrate and phosphate in excess. However, we know that for an aquarium to operate stably, within reasonable limits, it is necessary that the concentration of dissolved compounds in the water remains as stable as possible over time. A large volume water change will produce a sudden change that could destabilize the tank. For example, we know that performing water changes in an aquarium with a dinoflagellate infestation will significantly worsen the situation. Similarly, there is some likelihood that the chemical parameters of the new water will not match the existing ones, which can temporarily stress fish, corals and other invertebrates.

Even if the new saltwater has the same salinity and temperature as the aquarium water, there are other parameters that in most cases are different, e.g. pH, redox potential or dissolved oxygen. For example, it is common to see a drop in redox potential, pH and dissolved oxygen in the aquarium after a water change.

The transparency of the water is also changed suddenly after a large volume change. This implies an abrupt increase in light radiation that impacts fish, corals and other invertebrates, which can be detrimental depending on the magnitude of the increase experienced. Therefore, it is good practice to program the lights in acclimation mode prior to a large volume change.

Limitations of water changes
As one of the advantages of water changes is the reduction of nitrate and phosphate concentrations, in principle both equally (with some exceptions that we explain below), this could lead us to think that, by performing water changes of a certain percentage, with a certain periodicity, we could indefinitely maintain the nitrate and phosphate concentrations within the recommended ranges.

The reality is that, in tanks of medium or large volume, and depending on the organic load, water changes are not a sufficient tool to maintain pollution in acceptable values, without relying on other techniques such as mechanical/chemical filtration or denitrification and recycling of nitrate and phosphate by adding organic carbon. The reason is that too much frequently water changes of significant volume are necessary, which is neither practical nor economical. This does not mean that the changes are not beneficial, because they help to remove toxic metals, maintain ionic balance, salinity and trace element replenishment.

To illustrate this, figure 1 shows a simulation of the evolution in time of nitrate concentration (for example), starting at 1 mg/l, in a tank where weekly water changes of certain volume (10% to 50% in the graph) are performed. The scenario assumes an increase of 1.5 mg/l of nitrate per week (from nitrification). We can see that to stabilize a permanent concentration of aprox. 5 ppm, performing a 20% volume change per week, we need around 20 weeks! Figure 2 shows the number of weeks needed to reach stability versus the exchanged water volume per week with same assumptions.

Figure 1.png

Figure 1. Number of weeks needed to reach stability with water changes (versus % volume)

Figure 2.png

Figure 2. Weeks needed to reach stability versus % of weekly exchanged water volume.

On the other hand, when an aquarium has a very high phosphate concentration, phosphate precipitation occurs on the aragonite crystals of the live rock, in the form of calcium phosphate. This means that after a water change of for example 50% of the volume, due to phosphate concentration decrease, part of the previously precipitated phosphate will pass back into the water column, and the final concentration will not be half but higher. This phenomenon does not occur with nitrate.

Figure 3 shows a simulation graph of the number of changes required to reduce the concentration of any compound in the aquarium water (in percentages of 90% to 40%), with water changes of partial volume between 10% and 100%. For example, for a 60% nitrate reduction with 30% volume changes, 2.5 changes are required.

Figure 3.png

Figure 3. Number of changes required versus % volume.

Aquariums without water changes

Although it is possible to successfully maintain reef aquariums without water changes for months or years, it seems that the value of this practice comes more from a personal satisfaction than anything else. These types of aquariums require very detailed monitoring with frequent ICP analysis to verify trace element concentrations, which sometimes the ICP tests themselves are not able to determine with sufficient accuracy. The changes facilitate the maintenance of the ionic balance in a simple way, with replenishment of trace elements without the risk of overdosing, which in some cases can produce a real disaster.


New hobbyists entering the hobby wonder how often to perform water changes and what percentage. Obviously, it depends on each situation, but a good practice would be as follows: in the first months of operation, as long as nitrate and phosphate concentrations do not skyrocket, no changes are necessary, as the population of corals and other invertebrates will be small. The absence of water changes helps to stabilize the aquarium, especially during the first months where algae pests are frequent. Subsequently, rather than daily or weekly changes, which imply a greater effort, a monthly change of 10%-20% is recommended. It is well known that the fewer actions we perform in the aquarium, the greater the tranquility of the animals and the lower the probability of occurrence of destabilizing events.

There are many ways to perform the changes, both manual and automatic, using timers, level control or peristaltic pumps for continuous use. A rather convenient way is to stop the return pump and filtration devices, siphon all the contents of the sump and add new water. The return pump is switched on again and that's it. In this way it is not necessary to introduce any tubing into the aquarium, which can stress the fish.

In reference to water changes and other nutrient recycling and exporting techniques, the conclusion is that we should avoid allowing harmful compounds to reach too high concentrations, rather than making abrupt changes afterwards.
About author
Angel Cegarra is the author of the book “Reefkeeping Fundamentals”, where he presents a fascinating journey through the science and technology that govern life inside a reef tank. From nutrient recycling processes, through coral biology, project initiation, feeding techniques and maintenance of water chemistry. The book includes world class reef aquariums description along with priceless advice on how they were planned and how they are kept healthy and vibrant. Covering the biology of coral reefs and relating it to the biological, chemical, and physical parameters considered when creating a reef aquarium. Home aquarist moving into an area of knowledge that was once the exclusive domains of scientists.

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Pros: Excellent review of Water changes.
Cons: Some details that are listed as fact are still highly debated.
This was a really great article summarizing the pros and cons of water changes.

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