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What to test for and why

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What to test for and why

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.


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 - the simplest is to use either a Liquid Calcium Supplement or a Calcium Chloride Supplement, otherwise look into a calcium reactor, dosing kalkwasser or the Balling method.

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!


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 a mixture of magnesium sulphate and Magnesium Chloride at a ratio of approximately 1:7 (1 part sulphate to 7 parts chloride)

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.


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)

PH 8.0 to 8.3

PH will change over the day it will be highest at lights out and lowest just before the lights come on


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.

Only Ammonia 0 is acceptable


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.


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 coloured SPS corals to turn brown due to the increase zooxanthellae 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


Phosphates should be kept at undetectable levels in a reef tank. Anything above 0.03ppm will fuel algae growth and inhibit coral growth.

You should always run an Iron based phosphate removing media, preferably in a reactor, to reduce Phosphates. All the food we add to the tank contains phosphates; also tap water is high in phosphates, especially in rural areas, so good quality zero TDS RO water should always be used.


Iodine is a complex substance and the theories behind testing and dosing are just as complex. Rather than me try to explain this here in a few sentences, you need to read the following 2 links to fully understand the reasons behind testing and dosing.

For more information see here http://www.reef-eden.net/iodine_in_the_ ... uarium.htm and here Chemistry And The Aquarium: Iodine in Marine Aquaria: Part I — Advanced Aquarist | Aquarist Magazine and Blog


There is very little known about the importance of Strontium levels in a reef tank. Some hobbyists believe corals stop growing if levels are below 5ppm, however there is no real scientific evidence backing this up yet. Strontium is found in natural sea water at levels of about 8ppm, so we should aim to replicate the natural conditions our livestock has come from. Anywhere between 5 and 15ppm seems to be recommended. Most good salts should already have these levels and no further dosing should be necessary, however if you find that Strontium levels are below 5ppm then additives can be used at the stated dose on the bottle.


Testing and dosing Potassium is a relatively new thing in reefkeeping. It has become popular amongst expert hobbysists specialising in SPS corals using ULNS (ultra low nutrient systems). Some have identified that maintaining the correct levels of Potassium (380ppm) have increased colours in SPS corals significantly. You can use a Potassium chloride supplement to maintain and/or increase Potassium levels in your reef tank.


Many of us are under the impression that if we have diatoms in our tanks, then we have too high a level of silica.
Silica easily enters the aquarium through poor quality water, either untreated tap water or RO water with a higher than ideal TDS due to poor maintenance in changing the pre-filters, membrane and di-resin.
Silica is found in NSW at levels of around 0.06 to 2.7 ppm, we try to replicate all our other parameters to NSW levels, so why not Silica as well?
Silica is not only taken in by diatoms, but molluscs and sponges also benefit from it. Sponges utilise the silica to form internal structures, called spicules, which help them retain their shape. Molluscs such as limpets, utilise silica, for the growth of their teeth (radula), and a possible theory as to why such molluscs do not live long in captivity is the absence of silica - although still only a theory.
Reefkeeping is moving at a fast pace and I believe it wont be long until we actually see experienced reefkeepers dosing silica into their tanks.


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.


A question that crops up all the time amongst reefkeepers, is "What temperature is ideal for a reef tank?"

This is quite an open ended question as many corals available to us are collected from different regions. Typical reef temperatures vary from 20c to 40c depending on the area, depth and season. However, the majority of the corals we keep are found in shallow waters around the equator, typically these temperatures range from 24c to 29c.

A few things to consider when deciding on what temperature to maintain your reef. If using metal halide lighting which give off a lot of heat directed at the water, can you realistically expect to keep the water temperature at 25c during lighting periods without the use of a chiller and fans? Probably not. This may be easier to achieve if using T5 or T8 lighting, and probably very easy to achieve if you are using new LED technology.
So, aim for a temperature that you can maintain without large temperature swings between day and night which could stress your corals and lead to an early death.
If you are lucky enough to live in a warmer climate than the UK, then a chiller may be an essential piece of equipment regardless of your lighting choices.
Try and keep any high/low temperature swings to within 1c, if you can't achieve this, then consider increasing the running temperature before considering buying an expensive to run chiller unit.

Clip on fans can make a great deal of difference when trying to cool the tank and they can also aid in dispelling co2 which will in turn increase the waters pH.
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