Using Monitors – Using a Salinity Monitor in a Mediterranean marine system

 by George J. Reclos (this article appeared in the  FAMA issue in December 2002)

It is true that effective fish keeping isn’t necessarily linked to the use of zillions of gadgets – on the contrary, it is usually the sensitivity, knowledge and experience of the hobbyist that makes the difference between a successful tank and a failed one. However, sometimes, there are some little things that can make a difference. I recently had an experience which supports this view. Since I am relatively new to the marine part of fish keeping you may take these notes for what you think they worth – although most of it is applicable in freshwater tanks, too. Since I have decided to go on with keeping Mediterranean species I am always faced with two factors that I am not really familiar with. The first is the need for standardized and almost constant / stable water parameters and the second is the lack of adequate information about the species I decided to keep. I can’t do much about the latter. I have come to accept is as a fact and I am trying to fight it with lots of reading, a good deal of guessing and tons of observation. Observation of my tank and – more important – observation of the fish in their natural habitat. About the former – the “standardized” water parameters – I am trying to be as close as possible to what has proven to work for me. I don’t know if those are the correct parameters for the fish I keep or just a “lucky” combination that simply works for me but as they say “if it works, don’t fix it”. I tend to believe that since it has been working for 7 months now it can’t be just luck. Parameters like temperature, pH, alkalinity, the concentration of the “N” compounds (ammonia, nitrites and nitrates), calcium and salinity are closely observed and notes are taken on a regular basis. Chillers with electronic sensors take care of the temperature (quite effectively I must say), a pH monitor gives me the correct measurements and – till recently – the use of a glass hydrometer was supposed to give me the specific gravity of the water. The key words here are “supposed to”. I have tried as hard as possible to treat the two tanks in exactly the same way, adding the same things, making the same water changes at the same time using the same water etc. All in all, I was pretty sure that the two tanks should have almost identical water parameters. The pH was almost the same, the “N” compounds are almost at negligible levels and the hydrometer was showing a difference of 0,001 in specific gravity (1,027 vs. 1,028). However, I had observed that fishes in the left tank seemed to really thrive (the Salaria pavo “Big Daddy” has almost doubled in size in a mere 5 months and my Symphodus ocellatus has gained size as well as color ) while in the other tank – apart from some casualties (fortunately very rare) - fish seemed to be more lethargic, not acting the same way (I am referring to the same species). Moreover, some other species (like the crab) decided to move from the right tank to the left tank. At first I thought it was by accident, just a lucky and amusing incident and transferred the crab back in the initial tank. The next day it had changed tanks again. What really puzzled me is that it never tried to move from the left tank to the right one  – during the last four months anyway. While browsing the net I came across an electronic salinity monitor which didn’t seem very expensive to me, so I bought it. After carefully calibrating it with the solution supplied I took a measurement of the water in both tanks. The monitor has a temperature sensor which compensates the salinity values for temperature giving the results in milli-Siemens which are subsequently expressed in ppt (parts per thousand) or specific gravity (you have to use the table supplied with the monitor). I am sure you can all guess what the readings showed me. Firstly, the salinity was considerably higher than what I thought. Secondly, the difference between the two tanks was not a mere 0,001 but 0,002+. Actually the left tank showed 58 mS (or 1,029 – so much for the 1,027 I thought I had) while the right tank showed 61,5 mS (or 1,031). Measuring the pH revealed more significant differences. In the “loaded” tank the pH was 8,32 (down from 8,42 a week ago) while in the algae growing tank it was 8,40 (not changed). Naturally, the next step is to watch the fluctuation of those parameters for a couple of weeks and then gradually bring the salinity of both tanks to the same value as that of the sea. Needless to say that I had to wait anxiously for one week till I had the opportunity to go to the sea again to take a measurement of the salinity and the pH of Mediterranean water in situ. As expected, books may say some things but you have to take the measurements yourself. I will not even mention the values I obtained testing water found in tide pools you will be shocked. Salinities up to 1.036 and temperatures up to 32oC (!!) I collected water 5 meters away from the coast and the values obtained were the following: salinity 65,7 mS (specific gravity 1.032) and pH 8,27 (those interested in keeping Mediterranean species take a good note of those values). It seems that the high rate of survival in my tanks was only due to the fact that most of the species I collect come from tide pools which have extremely varying conditions therefore the species that inhabit them are able to adapt to a wide range of parameters. Be that as it may, the use of a salinity monitor is highly recommended – just to be on the safe side. A week after, I was amazed to see that, although part of the water was removed by the skimmers and was only replaced with conditioned tap water, salinity didn’t go down but climbed to 59,7 mS and 65,2 mS for the left and right tank respectively (specific gravities 1,0296 and 1,0315 respectively). Which makes a very good point for using R/O water in marine systems. A new measurement of pH (no water changes in the meantime; just adding tap water to replace the water evaporated) revealed that the “loaded” tank’s pH was even lower now (8,21 – closer to the optimum value) while the algae growing tank (the one with the “problems”) was a bit up (8.49).

After playing with my chemicals, I left both tanks in a “transition” state on their way to the optimum values (now that we know them). Both tanks have a pH of 8.30 and a salinity of 1.031.  

It is evident from the above that the use of monitors – especially in marine systems but not limited to them - is a must. The accuracy of a well calibrated and conditioned device far surpasses that of the test kits or the hydrometer. You can take as many measurements as you like, as frequently as you like, in as many tanks as you have and get a clear picture of what is going on there. The “range” – type results of most test kits (e.g. pH measurement between 8.2 and 8.4) can tell you very little – if anything at all. It may be OK for some tasks (e.g. to know if you have the alkaline water needed for African rift lake cichlids) but you can’t count on them if you keep species that require far less fluctuation in their water parameters (like reef systems). In fact if I used them in my tanks everything would seem to be rock stable and identical. Moreover, the interpretation of the results in neighboring ranges (kits using color scales) is subjective and sometimes difficult. When it comes to the “multi-parameter” dip sticks, things are even worse. In fact, I only use those sticks if I observe fish behaving strangely in a tank to get a gross idea of what might be going on and – more important – which are the parameters that need to be analytically tested first. As I always say, time is critical in such cases, so testing for the correct parameter first, may well make the difference.

Choosing a monitor is not a difficult task since there are many manufacturers offering relatively cheap and good quality monitors for a range of parameters. Some manufacturers even offer control “stations” which can take as many as eight probes on them. The usual stand alone monitor is divided in two parts, the monitor itself (which has a display where the value is shown) and the probe which is submersed in the water. Some manufacturers also offer monitors which are “one piece” and are therefore used as a “probe”. Depending on the parameter measured, monitors have a first or second digit  accuracy. As far as pH is concerned this is 10 times more as compared to the usual pH color scale kit.

One very important thing when using monitors is the calibration and the conditioning (you may call it storage). If these two stages are overlooked or done without any caution, then results will not be accurate. Following the instructions of the manufacturer is a must while you may also add your knowledge from other similar systems. Calibration – despite what the manufacturer says – is better done every time you plan to use your monitors (once, not before every measurement) and should be done just before using it. I use the standard pH solutions we use in the lab to calibrate my pH monitor (the solutions sold by the monitor manufacturer are very good, too) and I prepare my own calibration solutions for the salinity monitor. I also prepare my own “conditioning” solution for the pH monitor. The essential thing to remember for the pH monitor is that the probe should never be allowed to become dry. Using plain water to keep it moist is not a very good idea either. The best solution is to use a saturated potassium chloride solution (KCl) which means approximately 4 M (30 grams of KCl / 100 ml distilled water). This will keep the probe in prime condition for at least a couple of months if you use a watertight cup filled with some cotton and the conditioner over the tip of the probe. Always rinse the probe in tap water before using it. If you observe the accumulation of salts in the end of the probe do not try to scratch it away. Just use tap water. Those salts are extremely soluble so they will go away easily.


The calibration solution for the salinity monitor is easier – all you have to do is to prepare a 3,5% solution of sodium chloride (NaCl) by dissolving 3,5 grams of it in 100 ml of distilled water. This results in a solution with a specific gravity of 1,026 and a reading of 53 mS.

A final note for those wishing to play the little chemists: using distilled water is essential when you make conditioning and - most important - calibration solutions. The use of de-ionized water is not recommended. This is because the de-ionized water (in contrast to what is generally believed) is not ion-free. What happens is that the bivalent cations and anions (calcium, magnesium, iron, sulfates, phosphates etc.) are “exchanged” with sodium and chloride ions respectively. Thus, this type of water already contains sodium chloride and is not recommended for use in the preparation of “critical” solutions like the calibrators. If you have a good quality R/O unit you can use this water with very good results. If not, you can go to the pharmacy and get a bottle of “water for injection” (WFI for short) which is of pharmaceutical grade. It is cheap, readily available and one liter will allow you to prepare enough calibrators and conditioning solutions for a year.

Some more hints for a correct measurement.

  • Always use vials to take water out of your tank and make the measurement out of it.

  • Don’t just drop the probes in the tank – unless your house is sterile, which it probably isn’t.
  • Do not return the water in the tank after the measurement (for the same reasons).
  • Use water from your tank to wash the vial a couple of times before taking the measurement. This will dissolve and remove salts remaining there from previous measurements.
  • After the measurement is over, wash the vial with tap water and wipe it dry with a kitchen paper.
  • Do not let it dry in the air as the salts will stay in the vial.
  • Always store your vials with their openings facing down – this will not allow dust to get in them.
  • It is better to use one vial for each type of measurement (one for the pH and another one for the salinity or conductivity or whatever). You may label your vials accordingly to avoid mixing them.
  • If the measurement you are taking is related to temperature (for example both pH and salinity are) consult the manual which came with your monitor.
  • If there is a temperature thyristor incorporated in the probe wait long enough for it to reach the temperature of the water (this can be quite a task, especially if the water is much cooler or warmer than ambient temperature).
  • Immersing the probe in the tank water you use to wash your vial before the reading will help it to reach the correct temperature sooner.
  • Usually pH measurements are taken at room temperature. In any case you should have your calibrators and tank water at the same temperature for accurate measurements. One way to do that is to let both the calibrators and your tank water rest for half an hour in the room so they both reach the same temperature.
  • Clean everything with tap water (better still : distilled water) after using it and store it properly.
  • Half the value of your monitor is the probe and you can’t imagine how easily they get destroyed if improperly stored. Once I had a probe destroyed by algae growth – it was used as a permanent monitor.
  • Keep in mind that probes will not last forever anyway. Usually they need replacement after 18 months – provided they are handled and stored the correct way. Inability to do so may result in a much shorter time of useful probe “life”.
  • A good indicator that there is a problem with the monitor’s probe  is the time it takes to be calibrated. If you are sure you are using the correct calibrators but the monitor will not calibrate, it usually means it is worn out and must be replaced. Never “push” it to work – results are not to be trusted.
  • A further note on this is to check the batteries. If you see the “battery low” signal on the monitor, it is highly recommended to replace the batteries with fresh ones before using it.
  • Always make sure that there are no air bubbles in contact with your probe – this will result in wrong measurements. This should be taken care of, even if the instructions don’t mention it.
  • Wait till the reading has stabilized. Values will change quickly at first and more slowly later but as long as they change you should let the probe in the water sample. A measurement can be regarded as “final” if there is no change for 30 seconds. If you are not in a hurry, I recommend 1 minute.
  • When measuring, place the monitor as far away from the vials and – if possible – at a higher level. Although probes are meant to be immersed, most monitors are not water resistant and will get destroyed if water comes in them.
  • If applicable, disconnect the probe from the monitor and store them separately.
  • Finally, always have a set of spare fresh batteries in stock. Keep in mind that batteries lose their power even if you don’t use your monitor at all. Always use alkaline type of batteries. If you do not plan to use your monitor for a long time (more than 2 months), it is better to remove them from the monitor.

I hope that this article managed to show you the importance of the use of monitors for accurate measurements and will allow you to have years of trouble free measurements.

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