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Water Testing

Updated: Feb 15


This is intended to be a short version of why reef keepers perform certain tests on their tank water without lots of the long scientific explantations that can be confusing. Let me know if I can help or make improvements to this blog.

Why do I need to test?

When any aquarium is new it can take a while for the water chemistry to settle so it's important to test to see what stage you are at. What you are looking for is STABILITY !

"nothing good happens quickly in a reef tank"

You may need to add some important chemicals to achieve stable water but water tests are what are necessary to determine this. There are a great variety of additives available from numerous manufacturers. I would say as a gerneral rule it's best to stick with one brand, as the additives from each company will be designed to work together and the instructions will be easier to follow.

As always if you're unsure I'm always happy to help, drop me a message

What should I test for?

When starting into the world of reef tanks these are what I would recomend testing for.

Alkalinity / Carbonate Hardness ( dkh )

Calcium ( C )

Magnesium ( Mg )

Nitrite ( No2 )

Nitrate ( No3 )

Phosphate ( P )

Salinity ( sg )

Why am I testing for these things?

I'll give a very brief explanation here. There are endless volumes of information advice and opinions available in magazines, blogs, social media and online forums. This is just a very quick intro.

Alkalinty, Calcium and Magnesium

Always test all 3, these elements affect each other so the level of one will affect the others.

Alkalinty ( dkh )

This is a measure of the alkalinity in your aquarium expressed as dkh.

If dkh drops to low your Ph can drop ( the water becomes to acidic )

Thisd is important as all life likes to live within a certain Ph range

Calcium ( C )

Marine organisms use C as they grow so it needs to be replaced

It's important for algae, microbes and crustaceans ( everything really )

Too much C can reduce your Alkalinty

Magnesium ( Mg )

Mg is used by living organisms to assimilate the Calcium they need

It also helps keep Calcium and Carbonate levels stable

If Mg is to low you won't be able to maintain stable alkalinity or Calcium

Nitrite, Nitrate and Phosphate

These 3 are all organic compounds, generally coming from food added to your tank

Nitrite ( No2 )

Created from food breaking down it's normally present in a new aquarium

It is toxic even at low levels

Don't add any corals until it is undetectable ( 0ppm )

Nitrate ( No3 )

Created from No2 as food breaks down

Corals and other life use it as food

High levels are toxic and can cause algae blooms


Generally comes from food added to the aquarium

Essential as a food source for corals and other life

High levels are toxic and cause algae blooms


This is how salty your water is.

I use a refractometer ( There are specific ones for Marine aquaria )

As water evaporates your water will become more salty so you'll need to replace it wit RO ( Reverse Osmosis ) water.

What test result am I looking for?

This is an area that can cause huge anxiety particularly for new reefers. All I can say is don't panic over it! Remember these two things

"nothing good happens quickly in a reef tank"


It's much better to have stable water chemistry than be constantly trying to change it to a magic number by pooring a new product into your water. If you need to alter anything it must be done slowly as you can pretty much guaruntee that altering the reading for one thing will alter readings for other things as well.


Anywhere between 7-11 dkh is generally recomended

I like to keep it at 8- 9 ( it does vary, I just need to add a little more carbonate to bring it back up over a few days )


400-450 ppm is generally recomended

I like to keep it around 430 ( if i need to add extra I do it slowly as it also alters the alkalinity )


1200-1400ppm is generally recomended

I aim for around 1300pm ( as always if mg need increasing I do it slowly over a few days )



I only test for this when setting up a new aquarium. Don't add any corals until it is 0ppm


5-20ppm is generally recommended

I like to keep it between 5 and 10ppm

Water changes should be enough to keep the level down



I don't add any phosphate to the aquarium as it gets enought from the fish food.

If it gets to high reduce the amount of food added and increase your water changes


1.025 Specific Gravity or 35ppt

Don't be tempted to use the cheap swing needle devices to mesure sg as they are very unreliable. I use a refractometer but now there are several electronic gadgets that can measure it for you and then even add some top up water for you.

What test kit is best?

I think the main thing to realise is that you must get kits that are intended to be used for marine aquaria, as freshwater kits tend to be different. There are many brands and they all seem to be pretty good nowadays. Just take your time and very carefully follow the instructions.

Can I get away without doing all these tests?

This is all sounds like a pain in the arse right?

Yep it is a hassle especially when life in the outside world intrudes on our time.

But it really is necessary to keep you water in check. Changes can happen very slowly and a visual check won't necessarily show a problem is coming until it's too late.

There are now companies that'll prefessionally test your water for you in a laboratory

( ICP tests ). They test for dozens of elements that a few years ago we aquarists had never heard of but it turns out are important for marine life. Your results are emailed to you with recomendations on any alterations/additives needed.

I'm not recomending you don't do your own water testing, but I think the ICP tests are a very helpfull tool and are far more accurate than we can achieve at home. With the added benefit of testing for elements it's not even possible to test for at home.

Ok, so that's a basic run down of testing !

Let me know if I can hep further and drop a comment below, I love to hear from you about your experiences are any tips or tricks you may have.


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Great article, good information

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Great article!

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Thanks very much.

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Great read

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thank you

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Great article

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Thanks you

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Great advice thank you

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