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What We Should Test For & Why Part 1



NSW levels average around 35ppt. This is what we should be aiming for within a reef tank. However areas such as the Red Sea have higher than average levels at around 40ppt. If you are running a Red Sea biotope, then it’s acceptable to have a salinity of between 35ppt and 40ppt.

However, for a normal mixed reef tank you should aim for 35ppt, anything lower than 32 or higher than 38ppt may induce stress upon the inhabitants.

To correct a low salinity level, use salt water made up to 35ppt for your top up water (dripped slowly) until the aquarium level has reached the correct level.

To correct a high salinity level, remove a qty of salt water and replace with fresh RO water (dripped slowly).

Whatever changes you make to salinity, they should be made gradually, do not increase/decrease salinity levels by more than 1ppt per 24hours.

As for equipment for testing, DO NOT use a swing arm hydrometer, they are usually very inaccurate and unreliable. Either use a refractometer or a high quality calibrated hydrometer (if it is under £30 then its probably no good!)

You can buy reference solutions for calibrating a refractometer which are usually made up to 35ppt, but you can also use RO or de-ionized water to give just as accurate a reading which should be zero on the scale. In fact you can even use tap water, run the tap for a minute or so to clear any debris and make sure it is about 25c, this should also give you a zero reading, compare it against RO water to be sure, but as we are talking parts per thousand not millions like with a TDS meter, even a tap water of 300TDS is not going to effect the reading which as I say is in parts per thousands, not millions.

For more in-depth information: Refractometers and Salinity Measurement


Alkalinity (dKH) should be maintained close to or just above NSW levels (7dKH) Aiming for 8 to 10dKH gives you a bit of a buffer zone.

If Alkalinity levels are low then pH levels can fluctuate, stony corals will stop growing, as will calcareous algae’s like coralline.

To increase Alkalinity levels, dissolve a teaspoon of bicarbonate of soda in a glass of RO water and drip into the tank. Test daily and continue this process until you have reached the desired level.

To decrease Alkalinity levels, this will drop on its own if you are not adding any additives and your fresh salt mixes are lower in Alkalinity than your current aquarium level. You need to test a fresh salt mix to establish the Alkalinity level, if it is high, then consider changing salt brand or stop water changes until the Alkalinity level in aquarium has dropped to the desired level.

For more in-depth information: Calcium & Alkalinity


Calcium should be maintained at between 390 and 450ppm, preferably bang in the middle at around 420 to 430ppm.

Many corals require calcium in the form of calcium carbonate to build their skeletons and grow. If calcium levels are below natural sea water levels, stony corals will stop growing, as will calcareous algae's like coralline.

There are several ways of increasing calcium levels - calcium reactor, dosing kalkwasser, using an "off the shelf" liquid additive, to name the 3 most common methods.

If your calcium levels are unusually high, do not panic, it will drop naturally if you are not dosing any calcium supplements and assuming your fresh salt mix contains a "normal" level of calcium. You should always have a fresh batch of salt water tested to check for any abnormalities in the salt, it's not unheard of to have a new bucket of salt containing as low as 300ppm calcium, or indeed as high as 550ppm!

For more in-depth information:Calcium $ Alkalinity


Magnesium should be maintained at between 1250 to 1400ppm, ideally at 1300 to 1350ppm.

Magnesium is important as it helps keep in balance calcium and alkalinity levels. If Magnesium is low then this will in turn result in fairly rapid changes in Alkalinity and Calcium.

If your Magnesium levels are low, you can increase it using an "off the shelf" liquid additive, or using a mixture of Epsom Salts (magnesium sulphate).

If your magnesium levels are high, then test a fresh salt mix to see what the mg levels are, if unusually high then consider changing your salt brand. Carrying out regular water changes with a salt containing less magnesium than your tank level, will gradually bring the magnesium levels down.

For more in-depth information: Magnesium in Reef Aquaria


Many aquarists complain about "low pH" in fact probably more than any other parameter!

It is a complex issue and I'm not going to go deep into it here.

Curing low pH problems can be difficult, every tank is different in terms of size, stocking levels, flow, etc, so there is no one golden cure I'm afraid.

The first things to consider are:

- Good air flow around the tank, ventilation in the room.

- Water movement in the tank, especially at the surface.

- The use of macro algae in a sump or refugium, either lit 24/7 or reverse lit at night only.

Stocking densities, lots of fish breathing in a small space will result in a lower pH.

Is the Alkalinity too low? (Below 7dKH)

For more in-depth information see here: Low pH: Causes & Cures


Ammonia is highly toxic to marine fish and invertebrates; however, due to the large amounts of bacteria in the water, the ammonia is rapidly removed and therefore does not usually create a problem in most established tanks. It only becomes a problem if something dies, which can cause a spike in ammonia levels which can have a snowball effect resulting in more creatures dieing one by one until you have a total tank crash!

Any dead fish discovered should be removed ASAP.

My suggestion is to take some sort of corrective action if the total ammonia rises above 0.1 ppm.

Water changes can be a fine way to reduce toxic ammonia levels. As a rule of thumb, ammonia will usually drop by about the same fraction of water that is changed, so a 30% water change will reduce ammonia by 30%.

For more in-depth information: Ammonia & the Reef Aquarium


Nitrite, the step between Ammonia and Nitrates in the Nitrogen cycle, is actually less toxic to marine fish than many people believe. With freshwater fish it is a different story though, and as most of us have kept freshwater fish at some stage that is where many of us get our beliefs that Nitrite at any level is fatal, simply not true.

At levels as high as 300ppm or more some marine fish can still survive!

Having said that, you really want to keep nitrites at undetectable levels. In a mature aquarium that should not be a problem, but in a new aquarium nitrite will be present for a few days or even weeks, this is why slow stocking and patience is important in the first few months of setting up a marine aquarium.

If you detect nitrite in your aquarium, it is likely something has recently died, or a lot of things have recently died, like in the event of a prolonged power cut for example.

Testing for nitrite isn't necessary week in week out in an established aquarium unless you really feel you want to test it.

for more in-depth information: Nitrite & the Reef Aquarium


Fish can tolerate nitrate levels of up to 100ppm in some cases. However, corals and inverts are not as forgiving. Nitrate’s in a reef tank will fuel problem algae’s and cause highly colored SPS corals to turn brown due to the increase zooxanthella algae cells within the coral polyps. This in turn can also slow the growth of the coral.

Fish can start becoming stressed at levels over 50ppm and regular outbreaks in fish diseases can occur.

Acceptable levels within a reef tank are 10ppm, preferably zero.

To reduce nitrates, there are numerous things to look at, however the main reasons for high Nitrates are overstocking, over feeding, use of poor quality water for water changes and top ups, poor water circulation, in-adequate filtration or poorly maintained filtration, old sand, lack of water changes, etc, etc

For more in-depth information: Nitrate in Reef Aquaria

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