I would like to put focus onto the most important water parameters in the reef keeping hobby. Some of them can only be tested in laboratory, some of them can be tested by you.


1. Temperature


Corals can tolerate a little bit lower and higher temperatures, but they prefer temperatures between 24 and 26 °C. If you have a problem in your tank, check water temperature first.


2. pH


During the evening your show stopping sized Acropora colony looked just fine, but the next morning the tissue in the middle of the colony has stripped. What happened


Probably the most common reason is that there was not enough water exchange (too low flow) in the center of the colony.


Through metabolism, the pH decreased in the low flow areas of the coral. In addition, if the pH of your aquarium water drops too low at night, this can also contribute to the same problem.


pH is definitely one of the Top 10 reef tank water parameters. At least, it should be tested once a week in the morning and in the evening. Usually in between AM and PM a fluctuation of 0,3 pH is often observed, but this should be minimised as much as possible. If the pH drops below 7.8 at night, something in your system needs to be optimised. A good pH to aim for is between 8.2 and 8.4.


What you can do to improve the pH of your tank:


  • Keep a stable Alkalinity KH of around 7.5 – 8 dkh.
  • Ventilate your rooms well.
  • Fresh air from outside for your skimmer´s air intake something similar or use an cheato algae refugium.
  • Having good flow on the water surface helps with gaseous exchange too.


3. KH or Alkalinity


The alkalinity, carbonate hardness or KH is very important to buffer the pH of your tank. Carbonate (CO3) is also one of the major building blocks for corals to form their aragonite or calcite skeletons.


Problems when testing at home


  • As the solutions become older they become less effective, that leads to wrong readings.
  • The procedure can easy change slightly/errors occur giving inaccurate results… always test more than once at one time.
  • The color perception of you and your eyes can lead to wrong readings, try to read in indirect sunlight.
  • Sometimes the test is bad from the beginning.


4. Salinity


Everybody is asking for optimal water parameters to achieve the best results in reefkeeping. I say mother Nature knows it best.


If you know your salinity, you can easily find out which element concentrations fit the best. In our Hobby, a salinity of 35 PSU (35 g salt per kg water) seems to be the standard that most of us are trying to achieve.


5. Calcium


As already mentioned before, the optimal calcium value depends on the salinity of your tank water. I recommend to test for calcium once per week.


Higher calcium (+ 20 mg/l) and KH (9 – 10) values can increase the growth rate of your corals when all the other parameters (light, flow, pH, nutrients, …) are fine. But if you have a ultra low nutrient system this can cause problems. In that case I recommend to keep the levels (KH and calcium) on a natural value or slightly reduced.


6. and 7. Nitrate (NO3) and Phosphate (PO4)


Nitrate NO3 is the accumulative end product of the aerobic nitrogen metabolism process, which happens naturally within the aquarium from the waste products from fish etc. (nitrification, NH4 -> NO2 -> NO3).


If there is no nitrate in your tank your corals might be nitrogen limitated.


Nitrogen limitation can lead to decreased growth rates, worse polyp-extension and coral coloration.


In that case you should add some nitrogen in form of ammonium, amino acids or planktonic food.


Add it at a minimum of every second or third day.


If algae growth occurs, stop dosing.


Regular phosphate tests are only capable to measure PO4. Low phosphate (PO4) contents does not say a lot about general phosphorous availability.


There are more phosphorous containing molecules and particles in your tank corals can feed on. Remove GFO or similar media if phosphorous levels are low. If necessary add PO4 to your tank.


If nutrient levels are to high you will also get problems with your corals, algae growth cyano etc. Keep Nitrate below 20 mg/l and phosphorous below 40 µg/l (LPS, soft corals) and 20 µg/l (SPS).


8. Iodine


To keep the iodine levels stable, you need to find out what consumption rate your tank has. ICP can help you on this. Keep in mind that not only the organisms in your tank will deplete iodine, also activated carbon will bind serious amounts of iodine.


When you know the consumption rate, Iodine should be dosed on a daily base. I’d recommend to do it at least twice per week if you are lazy. Best source for iodine is IMO potassium iodide. In my experience iodine levels < 300 µg/l will be tolerated well when only potassium iodide.


9. and 10. Heavy metal pollutants


Magnet holders for circulation pumps are one of the best inventions for reef tanks in the past years?


If you have elevated levels of either or mixture of Tin, Nickel and Cobalt, the source might be an corroding magnet or some metal in or around the system.

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