API Saltwater Calcium Test Kit

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Aquarium Pharmaceuticals, API, MARS Fishcare

This kit allows for easy and accurate measuring of calcium levels to help in maintaining a healthy aquarium.

Why test for Calcium?
Monitoring calcium levels is essential for keeping a successful reef aquarium. Reef organisms use calcium to build a calcium carbonate skeleton. Over time, as the calcium in the water is utilized, the calcium level will gradually drop. Calcium must be present for corals to grow. Even reef invertebrates such as crustaceans, mollusks and encrusting algae, require calcium to grow and remain healthy.

Calcium levels drop as it is used by invertebrates or as it reacts with excess phosphate. Making a partial water change may help to maintain proper calcium levels, depending on the brand of salt mix. Special calcuim additives are also available. Elevated phosphate precipitates calcium, rendering it unavailable to reef organisms. Phosphate enters the aquarium from tap water and as a byproduct of fish and invertebrate metabolism. Use Aquarium Pharmaceuticals Phosphate Test Kit to test for phosphate. If necessary, use Aquarium Pharmaceuticals Phos-Zorb™ to remove excess phosphate. Phos-Zorb is a special phosphate-removing filtration media that comes in a flow-through pouch. It conveniently fits into any filter.

Carbonate hardness (KH), often referred to as alkalinity, is important for reef organisms. As carbonate levels drop the pH becomes unstable. The carbonate hardness level (KH) should be 7-10 dKH [125 – 179 mg/L (ppm)] to encourage growth in a reef aquarium and maintain pH. Use the Aquarium Pharmaceuticals KH Test Kit to measure the carbonate hardness. Add Proper pH 8.2 to increase carbonate hardness and stabilize the pH to 8.2.


Testing Tips:
The Calcium Test Kit reads the calcium concentration in milligrams per liter (mg/L) which is equivalent to parts per million (ppm).


  • Read thoroughly before testing. Do not allow test solutions to get into aquarium.
  • To remove childproof safety cap: With one hand, push red tab left with thumb while unscrewing cap with free hand.
    • Rinse a clean test tube with the water being tested.
    • Fill the test tube to the line with the water being tested.
    • Holding the bottle vertically, add 10 drops of Calcium Test Solution #1 to the test tube, cap and shake for 10 seconds.
    • Shake Calcium Test Solution #2 bottle vigorously for 10 seconds.
    • Holding the bottle vertically, add Calcium Test Solution #2, drop by drop to the test tube until the solution changes from pink to the blue end-point. Each drop is equivalent to 20 mg/L (ppm) calcium (Ca2+). Cap and shake the test tube between drops. Be sure to count the drops added.
      • A pink color should form after the first drop.
      • If a blue color forms after the first drop, the calcium concentration is 20 mg/L (ppm) or below.
    • As the end-point is approached, the color will usually change from pink to purple before finally changing to blue. After the purple color forms, only one or two more drops should be required to reach the blue endpoint.

Note: Reef aquariums should have a calcium (Ca2+) concentration of between 400 to 500 mg/L (ppm). To save time, rather than adding only one drop at at time, several drops of Calcium Test Solution #2 may be added simultaneously at first, then the final drops can be added drop by drop as the endpoint is approached.

For example, if you think that the saltwater being tested contains about 400 mg/L (ppm) calcium, you can add 10 to 15 drops of Calcium Test Solution #2 at once as indicated in step 5, then cap and shake. If the solution is pink, continue adding drops, one at a time, until the end point is reached. If the solution is blue, then too many drops were added and the test must be repeated, adding fewer drops initially or adding the drops one at a time.

  • Bottle #1:
    • Keep out of reach of children.
    • Eye contact: gently flush with water for at least 15 minutes.
    • Remove contact lens, if present, after the first 5 minutes, then continue rinsing.
    • Skin contact: wash thoroughly with soap and water.
    • If swallowed, contact a physician or Poison Control Center immediately.
  • Bottle #2:
    • Contains Sodium Hydroxide.
    • Causes severe burns.
    • Harmful if swallowed.
    • Keep locked up and out of the reach of children.
    • In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. If contact lens is present, remove after 5 minutes, then continue rinsing.
    • Wear suitable gloves and eye/face protection.
    • If swallowed, contact a physician or call a Poison Control Center immediately.
    • In case of accident or if you feel unwell, seek medical advice immediately (show label where possible).

Product Manuals & Documentation

API Saltwater Calcium Test Kit

Owner's Manual available for download in Adobe PDF file format

API Saltwater Calcium Test Kit

Additional Info 1 available for download in Adobe PDF file format

Manufacturer Info

Aquarium Pharmaceuticals Aquarium Supplies
Aquarium Pharmaceuticals (API) has been helping people take good care of ornamental fish for over 40 years. Their research and development scientists have advanced the care of fish with all-natural medications, stable bacteria for biological filtration, one-bottle test kits, consumer-friendly dosing caps and much more. They’ve received over 25 global patents for their breakthrough innovations in the aquatics industry. And they continue to look for ways to improve the fishkeeping experience for hobbyists. Research. Experience. Dedication to scientific, quality treatment for healthy fish. It’s what makes API the first choice of aquarium owners. API manufactures and sells hundreds of products across many different product categories, including aquarium and pond treatments, aquarium equipment, aquariums and fish nutrition.

30 Reviews



Experience Level: Expert

Great Test Kit

Nice and convenient and easy to use and read. My old probe busted so need a temp fix to check my levels and this works just fine. Not completely hundred percent accurate but not off by much.

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Experience Level: Beginner

Good Test

Its a good test for calcium

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Experience Level: Intermediate

4.5 stars sounds good!

Con-Bottle 1 and Bottle 2 are un-proportional for a Reef Aquarium. Other than that, readings "seem" accurate. It stays at 440ppm. 1 week later, drops to 400ppm. I test my tanks calcium weekly. It is a mixed reef, lightly stocked, 100 gallons. Now, i don't know if this is the accuracy of this product, or the miracle mud I have in my refugium that is supplementing my tank, but I have not seen any significant inaccurate calcium readings. Never goes below 360ppm. Heck, I haven't seen my calcium go below 400 yet! ALSO, how do people know that this calcium test kit is "off by 20ppm or worse"? If you have a way to find your tanks ACTUAL calcium to be able to compare this test kit's readings, please do tell. All the Salifert, Tropic Marin, Red Sea, etc. Calcium Test kits aren't too accurate either, according to other customers.

2 out of 3 found this review helpful

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Experience Level: Intermediate


Yes, this is not highly accurate. Yes, I'd prefer Salifert. But not that many of us need such accurate calcium readings...for most of us, a 20 ppm margin of error is acceptable. Does it matter for me if my Ca is 400 ppm rather than 420? The Salifert kit, which I prefer, is about 5 times as expensive. So, for now, I'm perfectly happy with this cheap, slightly inaccurate, kit.

0 out of 2 found this review helpful

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Experience Level: Intermediate

Aquarium Pharmaceuticals Saltw

It's easy to use two step process where the contrasting color change makes it easy to measure calcium.

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Experience Level: Intermediate

calcium test kit

i have been using this test kit for about 4 days now and it is very easy to use and directions are easy to read and under stand as well as this test kit is dead on everytime ive tested with it at a great price

0 out of 1 found this review helpful

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Experience Level: Intermediate

Good money saver!!

Great value, not quite as accurate as other tests that I have used, but works close enough to help keep my calcium up. Very simple to use with a lot less waste of the testing materials.

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Socratic Monologue from OXFORD, WI UNITED STATES


Experience Level: Expert


Whatever other virtues this kit might have, it is not at all precise enough for reef use. One reason for this is that the color change occurs gradually over the course of a couple (usually three) drops -- this corresponds to a 60ppm range wherein the actual calcium level could be. (For what it is worth, this kit gives the same results as a Red Sea calcium test kit at the point when the API kit *just begins* to turn color.) But even if you figure out exactly what point in the color change is the sweet spot, the best resolution this kit gives is 20ppm. Considering there is likely at least ~5% variance (that is, another 20ppm in a 400ppm sample) due to drop size, accuracy of vial filling, etc, the best you might expect here is to read within 40ppm of actual value.

2 out of 2 found this review helpful

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Experience Level: Beginner

Great test kit!

I would like more of bottle 2 bottle 1 has the same amount of fluid and you only ue 10 drops vrs like 24 drops for 500ppm calcium from bottle 2. Overall its very repeatable and a great deal.

2 out of 2 found this review helpful

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Experience Level: Beginner

Calcium test kit

Very easy to use. Color change in the tube when testing is simple to read.

1 out of 1 found this review helpful

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