Water testing in the reef aquarium.
With more than twenty different physical and chemical parameters available for testing in the reef aquarium you may be forgiven if you find the whole subject of water testing a little confusing.
Why do you need to test?
No matter what anyone, no matter how experienced, says, you can’t tell what’s going on in an aquarium just by looking. You need you employ a variety of methods to find out what’s going on in your reef.
Which tests should you carry out?
Broadly speaking you can divide the available tests into three categories: the every day parameters that need to be adhered to, parameters you may need to examine in the case of a problem, and parameters that could perhaps be described as aspirational, ones that might be looked at when trying to optimise an already successful reef.
The essential parameters that need to be monitored in an established reef are: Temperature, Salinity, pH, Calcium and Carbonates, Nitrate, and Phosphate.
With newly set up reefs you’ll additionally need to pay attention to Ammonia and Nitrite and, although calcium and carbonates are of less importance until corals are introduced, proliferation of coralline or calcareous algae can make significant demands of these two parameters.
How do you test?
Depending on parameter we can choose from chemical based tests, electrical probes, photoelectrical sensors, optical instruments, traditional thermometers, and hydrometers. Chemical tests tend to be used for chemical parameters; physical parameters may be measured by a variety of other means.
What can’t we test for?
Given the budget, or access to an extremely well equipped laboratory, there’s not a lot that can’t be measured. But for the average reefkeeper (plus the majority of public aquariums) there’re plenty of things that can’t be quantified, for example the majority of the seventy-odd trace elements that are found in saltwater cannot be readily measured and, even if they could, there may be little merit as these elements, by definition, are available in trace or minute amounts. Their concentrations will be maintained by a combination of regular water changes and the food introduced into the aquarium. Indeed, given the speculation about the accumulation of trace elements in the reef aquarium, the value of water changes is that they can also help dilute any such accumulation.
The more important factors that we can’t test for are biological or biological in origin.
We can’t test for chemicals released into the water as a part of the non-stop chemical war being waged in our reefs by corals and other invertebrates.
We can’t test for the biological oxygen demand (BOD) of our reefs outside of the laboratory. This is the total oxygen demand of all the creatures present in our reefs.
At what levels should the various parameters be maintained?
This is a tricky one. You can aim to replicate natural seawater levels or you can opt for levels that have evolved over the lifetime of the hobby. In reference books you’ll often find discrepancies regarding natural sea water parameters, although the sea can largely be considered stable there are variations. Variations in salinity, temperature, and calcium are widespread, changing with depth, season, atmospheric pressure, and proximity to land. Locally these can change over the course of the year. We are primarily concerned with seawater from tropical latitudes in the vicinity of coral reefs so the figures I’ve quoted reflect this. See: Reef Ramblings – Recommended Reef Tank Parameters.
Measured by traditional thermometer, stick-on external thermometer, or electronic probe.
A reef needs to be maintained at a stable temperature within a fairly narrow range. Water temperature in a reef tends to edge towards the high end owing to the amount of lighting used and, in recent years, there has been a tendency towards a higher than reef safe ambient temperature for a few weeks of the year during the summer months.
Generally reef aquaria should be maintained somewhere between a minimum of 21˚C and a maximum of around 28˚C. For most captive reefs a temperature in the region of 24 – 25˚C, plus or minus one degree, will work well.
Note: some aquarium writers, myself included, advocate slightly higher, more natural temperatures in line with actual reef temperatures (up to a maximum of 32˚C). Higher temperatures result in lower levels of oxygen saturation and increased metabolic rates, factors that need to be taken into account if you decide to go this route.
Measured using chemical based test kit or electronic probe.
This is affected by temperature, flow rate, total animal load, and surface agitation. This is an extremely important parameter that’s rarely measured. In practise, a well-designed reef featuring good gas exchange through surface agitation, circulation, and aeration, that’s not overstocked, will most likely have adequate levels of dissolved oxygen. Saltwater holds lower levels of oxygen than freshwater and the higher the temperature the less oxygen available.
Modes of testing: Hydrometer. Electronic probe. Optical instrument (Refractometer)
As with temperature, this is another parameter critical to the survival of the animals in the captive reef.
Best tool for the job here is the refractometer as it gives a direct reading of salinity independent of temperature.
Hydrometers do not actually measure salinity they measure density in terms of specific gravity – it is important that if you’re using specific gravity that you always state the temperature of the sample as density varies with temperature.
Another method used to assess salinity. This uses a probe and meter and is more commonly used in aquaculture. Although accurate, may be misleading if water chemistry is awry, as this is actually a measurement of total dissolved solids and may be indicating the presence of ions other than sodium chloride.
Measured using a chemical based test kit or electronic probe.
In simple terms this is a measure of the acidity or alkalinity of a solution. A pH of 7 is considered neutral, a number lower than this tells you a solution is acid, and a number higher tells you a solution is alkaline. The scale used is logarithmic so a pH of 8 is ten times more alkaline than a ph of 7, a pH of 9 one hundred times more alkaline. Electronic testing is far better than chemical test kits owing to the finer resolution offered.
Note: pH fluctuates over the course of twenty four hours in response to biological activity such as respiration and photosynthesis. Generally pH is lowest first thing in the morning before the lights go on, it rises over the course of the day, peaking at lights out in the evening. For this reason, always test at the same time of day so meaningful comparisons can be made.
Chemical based test kit. Measured in milliequivalents per litre (meq/L) or degrees hardness (dKH). 1 meq/L = 2.8 dKH.
This is the buffering capacity of seawater, its ability to maintain pH, resisting the trend towards acidification resulting from biological processes such as respiration. As alkalinity is a function of carbonates and bicarbonates, along with a few other compounds, it can also be used to evaluate the availability of carbon that, along with calcium, goes to providing the material for calcification in corals.
Modes of testing: chemical based test kit, electronic probe (accuracy and reliability not yet proven at hobby prices).
Calcium and carbonates should always be considered together. There is little point in testing one without the other and there is no point aiming for some particular value of Calcium without the Carbonates being balanced in the correct proportion required for calcification.
Chemical based test kit.
Magnesium has an important role to play in the reef aquarium in supporting the buffering system and in its effects on the process of calcification. Magnesium should be about three times the level of Calcium in the reef so, for example, if Calcium were at 400 ppm you’d expect to see Magnesium at around 1200 ppm.
Ammonia, Nitrite, Nitrate, and Nitrogen gas.
Monitoring the nitrogen cycle lets you evaluate how efficiently filtration is taking place. Both Ammonia and Nitrite are potentially lethal and should not be present in the aquarium. Once a reef is established there’s little to gain by testing these two parameters unless you are adding additional livestock or live rock, or if you’ve experienced the loss of a fish or other animal where you’ve been unable to recover the body.
In a newly set up reef monitoring Ammonia and Nitrite will allow you to judge when it’s safe to start adding livestock.
Chemical based test kit.
When raising larval marine animals it can be useful to employ an Ammonia indicator badge, especially as a larval rearing rarely features filtration. Note: Ammonia toxicity increases with pH. It can be a good idea when running larval systems with no filtration to maintain a lower pH to take advantage of this, as toxicity can then be negligible. Don’t try this in a reef!
Chemical based test kit.
Chemical based test kit.
A well set up reef should show little if anything in the way of Nitrate. Although not toxic, as with Ammonia and Nitrite, it’s necessary to monitor Nitrate to ensure the level stays low to help prevent algal growth, at the same time if you’re getting a zero measurement, it may well indicate that a small amount of nitrate needs to be added for the well being of your corals.
Chemical based test kit.
This is one parameter you certainly want to keep under control. In combination with elevated levels of Nitrate, Phosphate acts as fuel for unwanted algal growth. Additionally, raised levels of Phosphate inhibit calcification in stony corals.
When you use a good quality phosphate adsorbtion medium the only way you can tell if its capacity is exhausted is by regular testing for phosphate. If you reach a point where the level of phosphate is no longer being reduced, or, indeed is increasing, then that’s time to replace your phosphate remover with new.
Minor or Trace Elements.
Chemical based test kits.
Although there are test kits available for these, for some parameters the value of testing is questionable. Some test kits may either produce a result that is difficult to quantify or difficult to reproduce.
Testing of these parameters can be considered aspirational, as when you’re trying to fine-tune a reef to perfection (a questionable pastime) or, more usefully if you’re trying to perfect conditions for a particular difficult coral such as Goniopora species. In this example there’s been a certain amount of speculation regarding the importance of iron and Manganese in maintaining these corals.
These parameters may also come under scrutiny in a last ditch effort to sort out a particularly troubled reef when no other obvious causes can be found.
Boron is another element that contributes to the buffering system in saltwater. Salts that contain higher levels of Borate may give incorrect readings when standard Alkalinity test kits are used.
The jury is still out when it comes to the utility of adding Iodine to the reef aquarium. Some aquarium writers are against its addition, while others are in favour. Toxic if over dosed, it is important for the growth of many species of macro-algae and is found in the tissue of some soft corals. Iodine exists in a number of different forms in saltwater, which may be one reason that testing produces unreliable results.
Given the low levels of Iron in seawater the prospect of testing for it in any meaningful way is slim. However iron is important for algae including zooxanthellae, the symbiotic algae associated with photosynthetic corals.
Silicates shouldn’t really be a problem these days. Before we adopted the practise of using RO water to make up fresh saltwater and for evaporation top up, it was quite common for aquaria to suffer a bloom of brown algae (actually diatoms) after every water change.
RO membranes aren’t as efficient at removing silicates as they are at removing some other elements, consequently special, more expensive membranes are available that remove a greater proportion. This may not be a good idea, silicates are required for the healthy growth of sponges and diatoms, as long as they don’t proliferate excessively, are a useful source of nutrition for many reef animals.
Strontium is another element that’s in dispute over its application in the reef aquarium. Anecdotal evidence seems to support the addition of Strontium, yet there’s some evidence that present day corals don’t utilise it in the same way a corals from the geological record, an oft cited source for the chemistry of calcification. Natural seawater levels should be fine but there is a question over the accuracy of test kits available in the hobby.
Chemical based test kit.
Copper isn’t generally a test required for the reef aquarium. The reason for this test is to quantify levels of copper when it’s being used as a therapeutic for fish disease. Copper is toxic to marine invertebrates so can’t be used in the reef, if someone sells you some cheap rock to use in your reef that has previously been in a fish only tank, then I’d certainly suggest soaking the rock in for a week or so before testing whether it’s leaching copper from some previous cycle of medication.
Redox or Oxygen Reduction Potential (ORP) may be monitored for a couple of different reasons. When using Ozone, a redox probe and controller can be used to control the amount of ozone being applied, and prevent dangerous overdosing from occurring.
A more general usage of an ORP monitor is as an indicator of the overall health of a reef system. Observing a downward trend in the reading alerts you to the fact that conditions in your tank are deteriorating.
Chemical based test kit.
Gives an indication of the degree of pollution resulting from phenols and other organic compounds
Monitoring light output tells you when your lamps need replacing. My recommendation would be to record your light output at a fixed distance from the lamp (in the case of metal halide lamps after they’ve been run for around 100 hours) periodically re-check, then when output has reduced by 30% you’ll know it’s time to replace the lamps. When you do replace an aging lamp, always raise the new lamp to a point where you’re getting the same output as the old lamp to avoid photo shock to your corals, and then gradually lower the fixture over a week to ten days until you reach the original position.
As regards the amount of light used, this depends on the corals being kept and on their original depth on the reef.
The most suitable way of measuring light in a reef is to employ a PAR or Quantum meter, this instrument measures photosynthetically available radiation and can be viewed as quantifying the number of actual photons hitting a coral. Forget about Lux or Lumens, these units are biased towards human perception of brightness and are weighted towards certain sections of the spectrum and don’t give an accurate evaluation of light available to drive photosynthesis.
Turbidity or discoloration of the water can be closely linked to the above notes about photo shock in corals. If your water is discoloured and you respond by adding carbon to clear it you can end up with your corals suffering from photo shock.
This can be evaluated in a number of ways, from the high tech – PAR meter or other photoelectrical sensor, to the low tech – a strip of white plastic part immersed in the water and then visually comparing the difference in colour above and below the water line.
The measurement of bacteria in the home aquarium is something of a new concept. By using bacteria dip tests in marine aquariums we may be able to learn more about how our reefs are performing by seeing what levels of bacteria are present. Bacteria can have a significant affect on the total BOD and, while many species are benign, others may become pathogenic under certain circumstances. I’ve not yet applied this testing to the reef aquarium but I’m currently looking a bacteria levels in live food cultures and seahorse tanks to see if there is any correlation between the number of Vibrio species present and the incidence of bacterial disease.
Not just the aquarium.
It’s not just the aquarium water you need to test. It makes good sense to look at the water you use for either making up your salt mix for water changes or for evaporation replacement. Testing here serves the purpose of evaluating the efficiency of your Reverse Osmosis (RO) unit or the purity of RO water bought from a shop. Does the membrane need replacing? Are you introducing Nitrate or Phosphate into your aquarium along with your water?
It’s also worth looking at how your salt mix is performing. Is it at the correct salinity? Is there sufficient calcium in the mix? Is it providing a correctly balanced ratio of calcium and carbonates?
Incidentally, regarding RO water, aquarists often query whether RO water is acid – it’s not, it’s neutral, with a pH of 7.0 at 25˚c. The confusion arises from the fact that the pH probes used in the aquarium hobby are incapable of giving an accurate reading when used to measure very pure water.
Whatever method you’re using for testing, try not to get hung up about hitting particular values. All reefs are different and will all display slightly different water chemistry owing to their unique assemblage of animals. Further more take into account the resolution of your testing method; any reading you get will be plus or minus the resolution of the test used.
If your not testing water at the accepted natural seawater level of 35 ppt or at 25˚C your readings will be inaccurate for some of these tests as concentrations will vary with salinity.
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