Testing Saltwater

Accuracy and Precision

When testing concentrations of saltwater components – especially calcium and magnesium – you must first test the specific gravity precisely and accurately.  Being precise means you can measure the difference between 1.0230 and 1.0235.  Being accurate means the value you are reading is true. Using a certified glass laboratory hydrometer that doesn’t need calibration – like the type we sell – is both accurate and precise   Refractometers and plastic hydrometers can be precise but they cannot be accurate by definition but can be functionally accurate if calibrated.

Basically a small change in specific gravity will have a significant impact on concentration values.  If you test your calcium at 1.023 and the tank evaporates water to 1.024, your calcium will test higher while the actual amount of calcium remains the same.


 We use a Dip Strip for pH, alkalinity, nitrite and nitrate – very easy, inexpensive and accurate. We are not interested in a “number” just if it’s good or not. If you want accurate values, Sailfert is the best. 


There are 2 types of ammonia – free ammonia and ammonium. Free ammonia is what burns the fish’s gills. Fish will come to the surface and gulp air when the ammonia is high. Ammonium is always present and is harmless. Most test kits will show a minimum amount of ammonia and what you’re reading is actually ammonium. Once your tank is established, your test should read zero or the lowest amount depending on the test kit and further testing is unnecessary.

Sources of ammonia are dead corals, dead fish and extreme OVER FEEDING.


Nitrite is formed when ammonia absorbs oxygen – hence lots of water surface movement and a protein skimmer for large tanks is required. For each ammonia molecule, 2 oxygen atoms are added and 3 hydrogens (or acid ions) are released – this is why pH drops during tank cycling. Nitrite is poisonous to fish as is ammonia. Once your tank is established, your test should read zero and further testing is unnecessary.

If nitrites are present, then ammonia is or was there for the same reasons.


Nitrate is formed when nitrite absorbs one more oxygen atom. This is the final product of ammonia break down.  Nitrates are relatively harmless. Fish can tolerate nitrates over 100 ppm but they are subject to nitrate burn. Corals like zero nitrates but less than 20 ppm are fine. Plenums reduce nitrates to zero and are highly recommended. Monitor nitrates especially if you don’t have a plenum. Deep sand beds simply do not work. See Deep sand bed problem

OLD test kits can show zero nitrate when it is off the scale.


pH is a measurement of how acidic or basic your water is. Saltwater aquariums should test an average of 8.3 to 8.4. Your highest reading is at the end of the lighting cycle, about 8.6, and lowest when your lights turn on, about 8.1. This is because of CO2 is being lost (pH rises) while the lights are on (photosynthesis) and CO2 absorption into the water at night (pH drops).

Fish respiration is the largest contributor of CO2.


Alkalinity, in simple terms, is your overall water quality. Comprised of a complex buffering system of carbonates and bicarbonates, average seawater is about 175 ppm or 3.6 Meq/L. An aquarium with a reading of 360 ppm or 7.2 Meq/L is good for reef tanks.

Raising alkalinity levels too high will lower your calcium levels. It’s alkalinity’s job to keep the pH stable – when alkalinity is depleted then pH begins to drop.


Phosphates come in several different forms and because of this, only certain types can actually be tested for. Fish food is the primary source for phosphates. Phosphates are required for living organisms to exist. Therefore removing phosphates completely is not recommended nor is it even possible.

They do contribute to algae growth when combined with nitrates. 

BLUE LIFE now makes a Phosphate Rx that is a liquid and remarkable.  We have reef tested it at 4x overdose and it’s completely reef safe.  It’s much easier to used that a phosphate reactor.


Calcium is essential for many things. Corals with a skeleton, halameda algae, coralline algae, snails, crabs, shrimp to name a few. Calcium levels are really not as critical as you’re led to believe. Anything between 350 and 450 ppm is fine. If you’re adding a calcium supplement and your tests read consistent and your corals look good, then don’t try to push your calcium level higher. Adding alkalinity boosters will push calcium levels down. Adding too much calcium will push your alkalinity levels down.

Star Polyps won’t open if calcium is too low.

Careful! kalkwasser or Calcium hydroxide is not a very good calcium additive. When first mixed with water the pH is over 13.5. Drain cleaner has a pH of 14. After the solution settles it tests about 12. Adding kalkwasser too quickly raises pH and alkalinity and actually decreases calcium by precipitation. If you calculate the amount of calcium you’re adding per liter, its pretty low (600 mg/l) compared to other additives such as Sea Chem’s Reef Calcium (50,000 mg/l) and a lot more work.


Strontium is the reason coralline algae is red. Testing is not important. If your coralline algae starts to bleach or form green, your strontium is low. Add more strontium and coralline growth increases.

IODINE I-1 aka Iodide

Iodine is a necessary element in all reef tanks. Creatures that molt, like crabs and shrimp, especially need iodine. Xenia loves iodine too. Iodine in your tank is chemically known as iodide. Iodine is actually poisonous, iodide is not.

Lugol’s Solution is a fancy type of tincture of iodine and when you add it to saltwater, it loses its color as it changes from iodine to iodide. You have just spent a lot of money on what ends up to be sodium iodide – an additive of table salt which is cheap.


Magnesium is associated with calcium solubility – it is best not adjusted in any way.

Magnesium sulfate is the second largest component of salt mixes


All other elements and ions, although present in small quantities, are just as important. Water changes will correct any concentration adjustments.    

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