Expert advice 10 most important reef tank water parameters.

Today I would like to put focus onto the most important water parameters 

in the reefkeeping hobby. Some of them can only be tested by us in our 

Laboratory, some of them can only be tested by you and of course, there´re some overlaps.

Please do not wonder, sometimes a problem is caused by a simple 

parameter like temperature. Maybe you will think … ah boring … but if the temperature is to low and it is not in your focus you will lose corals 

for sure.

Number 1: Temperature

One day a customer called me and told me that he has a lot of problems 

with his corals, but the results of his professional water analysis were 

just fine. At the end of our conversation, it was clear that the heater 

was broken and that the water temperature was below 20 °C. Corals 

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.

Number 2: pH

What experienced keeper of Acropora corals does not know this situation. 

During the evening your beloved show 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 current) 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 water parameters. At least, it should 

be tested once a week in the morning and in the evening. Usually in 

between AM an 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 7.5-8dkh

•     Ventilate your rooms well

•     Get fresh air (from outside) for your skimmer´s air intake or connect 

ATI´s Carbo Ex to your skimmer or something similar or use an algae 

refugee (reversed day/night light cycle)

•     Having good flow on the surface helps with gaseous exchange

Number 3: KH or Alkalinity

Reefkeepers with problems usually ask in forums or social media 

platforms for help. How often have I read: My SPS are dying ... all 

values are fine ... KH 8, Calcium 420 mg/l, .... Since we´ve started 

with ATI lab we´ve often detected much lower KH values than expected 

from our customers.

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.

A lot of times customers are reading KH levels incorrectly at home. The 

test procedure in the Lab is easy, low concentrated hydrogen acid is 

dropped into the sample until the pH drops to 4.3. This point is 

detected by an pH electrode. For your tests at home, there are pH 

indicators used. They change their color around a pH of 4.3. In both 

procedures, the amount of acid is determined to calculate the KH.

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.

I recommend to test using ATI reference solution frequently, set by our 

laboratory to ensure correct results!

Number 4: Salinity

Everybody is asking for optimal water parameters to achieve the best 

results in reefkeeping. I say mother Nature knows it best. We´ve 

received a lot of water samples from around the world. Corals thrive 

well in waters with different salinities.

In 1865, Johan Georg Forchhammer conjectured that the ratio of major 

salts in samples of seawater from various locations was constant. This 

constant ratio is known as Forchhammer's Principle or the Principle of 

Constant Proportions. Forchhhammer's theory was proven correct in 1884 

by Prof William Dittmar following extensive analysis of sea-water 

samples (Wikipedia).

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. Natural element concentrations at 35 PSU are (depending on 

the source of literature):

Chloride: 19800 mg/l                    Sodium: 11035 mg/l

Sulfur (as SO4): 2700 mg/l          Magnesium: 1313 mg/l

Calcium: 421 mg/l                         Potassium: 408 mg/l

Bromine: 67 mg/l                          Strontium: 8.1 mg/l

Fluoride: 1.3 mg/l

You can easily convert these concentrations to a lower or higher 

salinity by calculating:  element concentration / 35 x the salinity you 

have

For calcium from 35 PSU to 33 PSU calculate:  421 mg/l / 35 x 33 = 397 

mg/l

or use the tool provided on our website 

http://atiaquaristik.com/en/?page_id=1372

We in our ICP testing service adjust all reference values to the 

salinity we have calculated, by using the concentrations of sodium and 

chloride (responsible for 86% of salinity of natural seawater). Your own 

salinity readings can differ from ours because of elevated or reduced 

major element levels.

Number 5: Calcium

Aragonite is the material our dreams are made of and aragonite is made 

of calcium and carbonate. I guess everybody expected calcium to show up 

on the list of the top ten water parameters. 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.

May I have your attention please.

The calcium home test is an easy to do thing, but there is one step in 

the test procedure that should be performed with care. The indicator 

that is used reacts mainly with calcium and magnesium. Therefore 

something is added in the first step of the test to precipitate the 

magnesium. You can see this when the sample gets cloudy. I recommend to 

wait 1 minute before you go ahead. Shake the vial well in the meantime. 

If this step is not performed with care, some of the magnesium will not 

precipitate and can react with the indicator giving a wrong calcium 

reading.

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.

Number 6 and 7: Nitrate (NO3) and Phosphate (PO4)

Everybody needs to eat. Even the coral that survives from a big 

percentage from the photosyntetical produced metabolites of the 

zooxanthella that live within their tissue. Organisms all need carbon, 

nitrogen and phosphorous to built up biomass.

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. The ICP breaks down every 

molecule to its atoms.  Therefore it can test phosphorous. If we do 

measure phosphorous levels below 10 µg/l your tank might be phosphorous 

limitated. Phosphorous limitation can lead to decreased growth rates, 

worse polyp-extension, worse coral coloration and death. 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). If you 

have to reduce PO4 levels use GFO or AL-based removers. Lanthanium 

containing products does work also, but we do not recommend using them if 

you want to test your water in a lab. We can detect phosphorous that is 

bound to lanthanum. In that case we can´t say anything about the real 

phosphorous availability in your tank anymore.

Number 8: Iodine

Now we are heading to the water parameters only ICP can measure.

Science do not know what it does exactly, but in my opinion corals need 

iodine. Corals seem to have more problems with high lighting intensities 

as well as pathogens when the iodine concentration is low. 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 recommend to do it at least twice per week if you are lazy. Best 

source for iodine is IMO potassium iodide. Lugol´s or PVP I do not 

recommend, because of their disinfecting properties. In my experience 

iodine levels < 300 µg/l will be tolerated well when only potassium 

iodide-containing products, like ours, was used as iodine source instead 

of Lugols etc.

Number 9, 10 and …: Heavy metal pollutants

Who of you agree when I say magnet holders for current pumps are one of 

the best inventions for reeftanks in the past years? I love them, but 

some of them are black sheeps.

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.

Sometimes you will not find it straight away because it's covered by 

plastic and you might think the magnet is protected from corrosion 

inside. With many thanks to the people from the UK who have supplied me 

with tons of pictures of algae magnets, pump holders, screws and rotors 

full of rust. Some of them had to open the plastic covers and other 

parts of equipment to find what messed up their tank.

If Zinc only, or Nickel and Zinc in combination are elevated in your 

results list, I recommend to check also the RO water (fortunately we do 

this for free when you send us a regular seawater test) and also the 

used sea salt mix.

Other sources might be frozen food (that was not well rinsed before 

use), water care products of poor quality (including calcium and 

magnesium salts), artificial stones, concrete and mortar.

The end

In a perfect world you buy something for your tank and it would always 

be of best quality and will not do harm to your coral reef at home. BUT 

there are a lot bad products out there that can easy have sources of 

pollutants. Some may not even be detectable on a ICP test.  Be very 

careful what you put in your tanks!

Every fluctuation you can avoid, will help you to get the reef tank you 

want to have. Stability is one key to success, but keep in mind that 

success in reef keeping does not only depend on water quality. Reef 

tanks also need also a strong (alternating) current, a good filtration 

and proper lighting to thrive.

Now we have a good understanding and control of keeping a successful 

reef aquarium the things that can be most damaging are natural pests, be 

vigilant it what you add to your aquarium.

Happy Reef Keeping!