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Reef Chemistry

Chemistry can be a daunting subject but is a very important one for keeping your corals healthy. Below are some tips based on questions we've had over the years.

Test kits:

Love 'em or hate 'em test kits are a necessity for running a reef tank. A couple of general points to bear in mind:

Most test kits should have an expiry date on them - using one that's expired is not worth it as it's quite likely to give you an inaccurate result.

The test kits commonly used are designed for hobbyist use - in other words there is a limit to their accuracy and you will find that different brands can give different results. There is also sometimes a variance between batches of a given manufacturer's kit so it's good idea to get a new one shortly before the old one runs out so you can compare the two.

As you get to know your corals they will give a good general idea of the quality of your water - corals never lie.


Salinity (how salty the water is) can be measured in 3 different units - milliSiemens (mS), salinity in parts per thousand (ppt) or specific gravity (SG). We recommend keeping your tank at seawater levels: 1024-6 SG measured at 25°C. Levels outside of this range will distort your chemistry and are not good for your livestock.

There is still a misconception that keeping salinity lower will reduce fish disease and parasites. This is not the case - or rather the level would have to be so low as to kill corals.

For measuring salinity a refractometer is a good investment as they are normally more accurate than the swing-arm type hydometers. The are not overly expensive, are quick and easy to use and will last for years.


Test kits are ok for this but are time-consuming to use and work out expensive. We'd recommend you get a PH probe instead which will give you more accurate results. They're quick and easy to use and last a long time though they will occasionally need calibrating depending on useage. Calibration fluids are inexpensive and available from a number of suppliers. A range of 8.0 - 8.3 ph is fine.


Nitrate is a good indicator of how well balanced the tank is in terms of nutrient import and export. In general we'd recommend keeping it at or below 2.5ppm and anything over 10 is going to cause some corals, especially SPS, to lose colour and as the levels increase so will algae.

Bear in mind that it's possible to get a 0 reading but have lots of algae - this doesn't mean there isn't a problem, just that the nitrate is bound up in the algae. For measuring nitrate we'd recommend the Salifert nitrate test as it is relatively inexpensive, accurate enough and easy to read.

There are a vast array of quick fixes, vendor 'methods' and 'systems' and a multitude of mysterious additives available for dealing with high nitrate levels - but ultimately nitrate control is all about balancing nutrient import with export and achieving that is the only long term answer to nitrate problems.


Nothing over 0.03 PO4 for corals. If itís higher you'll need to take action. Higher levels will increase algal growth and inhibit the growth of corals as it interferes with the calcification process.

Phosphate 'fluidizing' reactors are good here but we recommend using the iron based media (e.g. PhosBan) as there is anecdotal evidence that the aluminium based ones can irritate soft corals. You can use the media in a canister filter or filter sock but it does have a tendency to clump causing the water to go round rather than through the media so watch out for this.

Both the Salifert and Seachem test kits are good although as your levels decrease you may need one with more resolution such as the D&D phosphate test or the Hanna Instruments phosphorous checker.

Alkalinity - also referred to as dKH:

Test every few days initially to see how much your tank uses. Both the API and Salifert test kits are good. Fauna Marin also do a very accurate but expensive test kit.

Alkalinity results can be expressed in three ways: meq/l, ppm and dKH - the most commonly used is dKH. After a while you'll see what your tank consumes and how much you need to add - you then only need to test every week or two. A range of 8-12 dKH is fine though we'd recommend a level of 8-9 dKH - this is just above natural seawater levels (7dKh) and gives you a margin of error. Levels outside of this range can have severe effects on corals, especially SPS and will cause other problems in the tank.

As with many parameters stability is key - it's better to aim for a number in the range, e.g. 9, that you can reliably maintain rather than chasing higher numbers and have the level see-sawing up and down.


Test once a week initially to see how much your tank uses (but see below). A range of 400-450 is fine. After a while you'll see what your tank uses and how much you need to add - you then only need to test every week or two.

Calcium and Alkalinity - a balancing act:

These two are the hardest to understand and get right but are also fundamental for your corals. Once you know what your levels are you can then find out the two things you need to know to get them right:

  • Are they chemically in balance?
  • It's very common to get them out of balance - if so you'll be forever adding bottles of stuff and still be out of whack. Before trying to increase or decrease the levels you have to get them in balance. See the link below to a Reef Chemistry Calculator to calculate what each should be in relation to the other.

  • Are the levels too high or too low?
  • Calcium will separate from the water and look like a snow storm if you add too much. About 480 is the point at which this can happen. If itís too low then add some but you must also add the balanced amount of dKH to go with it for this to work. See the link below to a Reef Chemistry Calculator for how much to use.

    If dKH is too high (over 12) this can damage corals and mess with your PH. Do not add anything to try and reduce it if it's too high (unless you've got some insane level) - it will fall on its own. If it's too low then add some but you must also add the balanced amount of Calcium to go with it for this to work. See the link below to a Reef Chemistry Calculator for how much to use.

    When adding either Calcium or Alkalinity supplements leave at least a few hours before testing.

  • What to use to increase these two?
  • For Alkalinity you can use sodium bicarbonate from a chemist or supermarket (it's what's in most fish brand 'buffers') but, just as with the fish brands, you need to test and measure carefully and match additions with Calcium. Don't add too much at once as this can lower your PH. There's other chemical considerations here but see the links below for more information. For Calcium Seachemís Reef Advantage Calcium is good as it has some Magnesium in it as well. Other Calcium additives are ok but you have to know how much they're going to add to keep things in balance.

  • What method to use to maintain Calcium and Alkalinity
  • There are four options for doing this:

    • Use '2-part' chemical additives
    • There are many aquatic brands or you can make up your own from chemical supply companies. You must keep up daily additions and also keep the amount you add of each in chemical balance if you use this method.

    • Calcium Reactor
    • We use them on all our tanks and is our preferred method. Output is already balanced for you but can lower ph if overused.

    • Drip kalkwasser
    • Output is already balanced for you but can raise PH and must only be dripped in slowly as the solution has a very high PH.

    • Weekly water changes

    The last two methods may struggle to keep up if you have a lot of SPS that are growing fast. Both methods will work but may not be sufficient on larger systems - in which case we'd suggest a Calcium Reactor along with Kalkwasser to offset any drop in PH.

Further Reading

These links will open a new window.
Reef Chemistry fundamentals
Reef Chemistry Calculator