In April 2008, I carried out a comprehensive salt water study on all leading brands of salt available to hobbyists in the U.K.
For this to have been accomplished, I have to thank the members of Ultimate Reef for sending me their salt samples.

The majority of you won't care about how these test results were obtained and by what method, but there will be a few that are! So if you scroll to the bottom of the test results you will find full details of the test kits and batch numbers used, methods of testing plus any other relevant information.

Seachem Reef Salt
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.40
pH (Pinpoint) - 8.25
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate P - 0ppm
Potassium - 380ppm
Strontium - 0 - 3ppm
Calcium - 545ppm
Magnesium - 1425ppm
Alkalinity - 9.1Dkh
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Wave Optimum Pro Reef Salt
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.50
pH (Pinpoint) - 8.40
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate P - 0ppm
Potassium - 300ppm
Strontium - 16ppm
Calcium - 350ppm
Magnesium - 865ppm
Alkalinity - 8.8Dkh
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D-D H20
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.60
pH (Pinpoint) - 8.90
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate P - 0ppm
Potassium - 370ppm
Strontium - 4ppm
Calcium - 415ppm
Magnesium - 1350ppm
Alkalinity - 8.1Dkh
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Red Sea Coral Salt
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.50
pH (Pinpoint) - 8.52
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate P - 0ppm
Potassium - 340ppm
Strontium - 22ppm
Calcium - 405ppm
Magnesium - 1170ppm
Alkalinity - 8.6Dkh
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Hobby Marin
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.70
pH (Pinpoint) - 8.88
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate P - 0ppm
Potassium - 400ppm
Strontium - could not test as calcium was too low
Calcium - 300ppm
Magnesium - 1080ppm
Alkalinity - 7.2Dkh
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Korallen Zucht Reefers Best Salt
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.50
pH (Pinpoint) - 8.52
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate - trace
Potassium - 410ppm
Strontium - 0 - 3ppm
Calcium - 400ppm
Magnesium - 1170ppm
Alkalinity - 13.8Dkh
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Tropic Marin
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.40
pH (Pinpoint) - 8.28
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate - trace
Potassium - 400ppm
Strontium - 0 - 3ppm
Calcium - 350ppm
Magnesium - 1200ppm
Alkalinity - 12.8Dkh
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Tropic Marin Trade Salt
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.40
pH (Pinpoint) - 8.34
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate - 0ppm
Potassium - 420ppm
Strontium - 0 - 3ppm
Calcium - 340ppm
Magnesium - 1140ppm
Alkalinity - 12.5Dkh
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Tropic Marin Pro Reef Salt
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.40
pH (Pinpoint) - 8.20
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate - 0ppm
Potassium - 320ppm
Strontium - 0 - 3ppm
Calcium - 400ppm
Magnesium - 1050ppm
Alkalinity - 7.7Dkh
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Reef Crystals
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.50
pH (Pinpoint) - 8.32
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate - 0.03ppm
Potassium - 420ppm
Strontium - 0 - 3ppm
Calcium - 480ppm
Magnesium - 1290ppm
Alkalinity - 13.1Dkh
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Instant Ocean
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.40
pH (Pinpoint) - 8.34
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate - 0ppm
Potassium - 375ppm
Strontium - 0 - 3ppm
Calcium - 365ppm
Magnesium - 1230ppm
Alkalinity - 12.0Dkh
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AquaMedic
Temperature - 26c
Salinity - 35ppt
pH (Salifert) - 8.40
pH (Pinpoint) - 8.00
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
Phosphate - 0ppm
Potassium - 380ppm
Strontium - 0 - 3ppm
Calcium - 485ppm
Magnesium - 1050ppm
Alkalinity - 9.6Dkh

All salt water samples were made up from 500ml of Zero TDS Reverse Osmosis water passed through Di-Resin.
The RO water was tested for nitrates and phosphates before use and read zero on both, although the kits used, are designed for salt water testing.

Approximately 18g to 20g of salt was used per sample to 500ml of water, depending on brand. Mixed to a Salinity of 36ppt at 18celcius.
Samples were mixed until all salts had dissolved and the water appeared to be clear. This took between 5 minutes and 30 minutes dependant on brand.
Samples were then de-cantered into 250ml screw top airtight and food grade plastic bottles, left at room temperature for 24 hours, then floated for 30mins in water of 26 Celsius. Salinity was then retested and adjusted if necessary to 35ppt at 26celcius.

The test kits used were as follows:
Temperature - System 2000 Digital monitor
Salinity - Pre-calibrated ATC Refractometer
pH - Salifert and Pinpoint pH monitor with new and calibrated probe
Ammonia - Salifert
Nitrite - Salifert
Nitrate - Salifert
Phosphate - D-D/Merk High Sensitivity Kit
Potassium - Fauna Marin Kalium Test
Strontium - Salifert
Calcium - Salifert
Magnesium - Salifert
Alkalinity - Salifert
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Test Kit Batch Numbers:
pH - 1207-C
Ammonia - 1007-E
Nitrite - 0807-C
Nitrate - NO3-1 1007-B, NO3-2 1007-B
Strontium - SR-1 01567E, SR-2 02662E, SR-3 02774E, SR-4 402579E, SR-5 02669E
Calcium - CA-1 0507-K, CA-2 0607-C, CA-3 0707-C
Magnesium - MG-1 0308-A, MG-2 0308-B, MG-3 0208-P
Alkalinity - KH 0507-D, KH-IND 0607-E
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The following test kits were pre-tested (twice each) against a Fauna Marin reference solution, this is supposed to be pre-calibrated with the following levels:

Magnesium - 1295ppm
Calcium - 415ppm
Alkalinity - 6.6 Dkh
Salinity - 35ppt

Now here is a problem.
The Salifert Magnesium test read the reference solution at 1200ppm.
The Salifert Calcium test read the reference solution at 400ppm.
The Salifert Alkalinity test read the reference solution at 8.6Dkh

I also tested the Alkalinity test kit against Salifert's own reference solution for this kit, and that read 7.00 Dkh, the reference solution is supposed to be 6.5Dkh.

I also found an issue with the salinity of the FM reference solution. It is supposed to read 35ppt, but I calibrated my refractometer with zero TDS RO water, checked it several times, and the salinity of the reference solution read 34ppt.

You can pretty accurately calibrate a refractometer with tap water that has been run for a while, even if the tap water is say 300TDS, this is in ppm, not ppt, so at 300TDS it should still be assumed as zero on a refractometer.

So what does this all mean?
Due to the apparent error in the salinity of the reference solution and the big margin of difference against all the test kits, I'm quite convinced that the reference solution is short of some salt! Quite a bold statement, but this would explain the lower than expected salinity, calcium and magnesium, although I'm not sure about the alkalinity.

Anyway, I have given a figure for the actual test kit reading, you can choose to take that reading, or adjust as necessary to take into account the reference solution readings or take an average of the 2.
I would definitely recommend you subtract approx. 0.5Dkh from the Alkalinity readings in any case.
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Salt Batch Numbers:
Not all salts come with a batch number unfortunately, where one was available they are as follows:

Reef Crystals - 0732601
Red Sea Salt - 1302070037
AquaMedic - 4146
Hobby Marin - 50350
Tropic Marin Standard Salt - 35C66
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Price comparison:
I have taken prices from various websites, using the largest available bucket and calculated an average price per KG excluding delivery charges.
I suggest you shop around when buying your salt, as the prices varied quite a lot from one web site to another. Also look carefully at the shipping costs if buying online, this could add another £10 to the cost!

Tropic Marin Standard Salt - £2.24
Tropic Marin Pro Reef Salt - £2.41
Tropic Marin Trade Salt - £?
Reef Crystals - £2.07
KZ Reefers Best Salt - £2.65
D-D H20 - £2.43
Aqua medic - £1.72
Seachem Reef Salt - £2.64
Hobby Marin Salt - £2.00
Red Sea Pro - £2.50
Instant Ocean - £1.85
Wave Optimum Pro - £2.06

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Conclusion:
The results above should not be thought of being 100% accurate.
They were obtained by a hobbyist with hobbyists equipment - just like you have at home.
I have made every effort possible to ensure the results are as accurate as I could achieve with the facilities available to me.
All the salt mixes were tested with exactly the same test kit and in the same manner to keep things as consistent as possible.
You should remember that all hobbyists test kits will have a degree of inaccuracy, add to this the human input - me! the results may have been totally different if sent to a science lab.
Also as only small samples of salt were used, there is no guarantee that the buckets were mixed properly. Its possible some elements settle nearer the surface and some nearer the bottom. The only way of being sure is to mix the whole bucket and take readings from that. Unfortunately I don't have the space or funds to mix up 700 litres of salt x 11 brands!

I am not going to give my opinion on which salt "is the best". I know what appears to be the best for me, but you may have different requirements, maybe a fish only system and not a SPS reef tank. Maybe you like to run a higher magnesium, alkalinity or calcium than me, or even higher nitrates if you keep a tank full of big clams!

So, please do your research and make your own decision on which salt suits your needs best.
Don't forget that batches of salt will vary from one to the next, so if these tested were carried out again in a year from now, they could look quite different.

If you want to discuss these results further, please go to this thread

Muzzy