This is part of a series on Aquaponic Water Testing.

Calcium is essential for plant health. Deficiency symptoms include death of growing points, abnormally green foliage, premature shedding of blossoms/buds, and weakened stems. Blossom end rot and lettuce tip burn are usually more issues of inadequate transpiration than calcium deficiency.

Hoagland suggests a level of 200ppm. I aim for 100-150ppm.

Testing for Calcium

Testing for Calcium is done three ways: Titration, Colorimetry, or Ion Specific Probe.  Titration is the classical method, but YSI + Hanna have a colorimetric method available. The Horiba LAQUAtwin Ion Specific Probe provides fast results, but requires calibration at every use.

Some tests give results in Calcium Hardness (CaCO₃). Since we’re interested in results in ppm Ca²⁺, divide Calcium Hardness results by 2.5.

1. API Calcium test (Resolution = 20ppm Ca²⁺, dropwise titration) $6.29@Amazon / 75 = $0.08 ea
   The API test is listed for Saltwater aquariums, and if you call API they will tell you it can only be used on Saltwater aquariums. I have tested it with freshwater samples of known concentration, and have found that it does work, but is difficult to read. Many aquaponic systems that are not actively supplementing calcium will see the color change on the first or second drop. In order to see the change, look through the top of the test tube, and make sure you have a white piece of paper as your background. The color change is light pink to light blue, so I recommend looking for a change from “slightly warm” in color, to “slightly cool” in color.
2. Taylor K-1770 (Resolution = 4ppm Ca²⁺, dropwise titration)
   $27.28@Amazon / 22 = 1.24ea
   2oz Refills $15.65 = $0.17ea
   Taylor tests are very common in the pool industry. If you have a way of measuring 25ml of water, you can purchase the reagents  (R-0010, R-0011L,  R-0012) for as little as $15.65 at local pool supply companies. Retail stores like Leslies Pool will charge as much for the reagents as the kit would cost, so be careful. The instructions specify resolution as 10ppm CaCO₃, which is equivalent to 4ppm Ca²⁺. That’s 5x better than the API test, and this test is easier to read.
3. Hach Digital Titrator + 8204 (40 – 1600ppm Ca²⁺ )
   Titrator: $159@Hach +
   Reagent 8204: $49.95@Hach / 100 = $0.50 ea
   Results are giving in ppm CaCO₃. Divide by 2.5 to get results in Ca²⁺. Resolution is 10x higher than the Taylor K-1770 test, but that increase in resolution is unnecessary in non-research systems.
    + 
4. Hanna Checker HI758 (200-600ppm, 1ppm)
   $41.65 Meter + 15 tests.
   Refill $21 / 25 = $0.84ea
   Sadly, I mention this Hanna Checker mostly to dissuade you from purchasing it. Notice the minimum value is 200. Most of the time in Aquaponics, we will be well below that 200 number.
5. YSI Calcium Hardness Calcicol (0-200 Ca²⁺; MDL 2; ±2)
   $30.45 / 50 = $0.61ea OR 73.08 / 250 = $0.29ea
6. Horiba LAQUAtwin Calcium Ion Meter (40-4000 ppm) $352@Amazon + ($100sensor+ $44calibration)/1500 = $0.10ea The sensor on this Ion Meter should be good for approximately 1500 readings, assuming you rinse it with distilled water after every use. It also needs calibration before every test run. If you have a number of separate systems, this item will save you time and money in the long run.

Know of another way of testing for calcium in aquaponics? Let me know in the comments and I’ll update this post.