Calcium-To-Magnesium Ratio Calculator For Shrimp Keepers


Ensuring the right balance of minerals in your tank water is critical to successful shrimp keeping. The wrong amount leads to molting problems, poor offspring survival, and worse immune function. Negative effects can also be seen in aquatic plants, leading to serious problems with a tank ecosystem. There are two ways to ensure the right mineral content in your water.

#1 - Use a Remineralizer With Pure Water

The most reliable way to achieve the right amount and ratio of key minerals is to use water that has been purified via reverse osmosis filtration or distillation (RO/DI water), then adding minerals in using a shrimp-specific remineralizer. While there are many different ones available that work well, we recommend the Salty Shrimp brand due to our experiences and the wide availability of their products.

You and your shrimp deserve the best and Shrimply ExplainedTM is here to provide that. If you want to learn all the basics of the science of shrimp keeping, then check out SB101: Are Shrimp Right for You?. You can also stay up-to-date about the latest tools and shrimp keeping info by subscribing to our mailing list (sign-up below).

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#2 - Check and Adjust Tap Water Levels

If you use tap water instead of remineralized water, then it’s tough to know whether the right ratio of minerals are present, specifically calcium (Ca) and magnesium (Mg). You may have General Hardness in a healthy range but still experience molting problems (Click here for background information on GH in shrimp tanks). GH is one useful test that tells you if the right sum of calcium and magnesium are present, but GH does not tell you whether they are present in the right ratio. Your GH could be made up of 100% calcium and 0% magnesium, or the opposite, neither of which are healthy. The recommended Ca:Mg ratio is roughly 2-5 parts calcium for every 1 part magnesium by weight. This is shown by environmental data from streams where Neocaridina live naturally (5:1 Ca:Mg) and from testing the Salty Shrimp remineralizers (2:1 Ca:Mg). The remineralizer is used by a wide range of shrimp keepers and has been shown to work consistently well, so we generally recommend aiming closer to that ratio, as opposed to the 5:1 seen in streams in Taiwan, where molting success frequency has not been tested.

Tap water in the US has around a 3:1 ratio and the right total GH on average, so many shrimp keepers have success using tap water to raise relatively insensitive species like neocaridina (cherry shrimp), ghost shrimp, vampire shrimp, bamboo shrimp, and amano shrimp. That being said, if you don't test your water then you are taking a risk. The only way to find out if your tap water actually supports the shrimp you want to keep is to either:

  • Check if your County Water Quality Report provides calcium and magnesium levels (search "[county name] water quality report in Google")
  • Put shrimp into an established tank and see if they have molting problems or not
  • Test your water to determine GH and Ca:Mg ratio, then adjust accordingly

If you experience molting problems or want to test your water before adding shrimp to be careful, then the following procedure explains how to do so. The rest of this article explains the most cost-effective way to test your tank's calcium and magnesium levels, then how to calculate the Ca:Mg ratio. If the ratio is far off of the recommended 2:1 ratio, then we also explain how to fix it by adding the missing minerals.

Step 1: Determining Calcium Concentration of Freshwater Tanks

Please note that the API Calcium Test and most others on the market are intended for saltwater, which typically has much higher calcium levels than freshwater. These tests typically only measure to the nearest 20 mg/L but the instructions below modify the testing procedure to double or quadruple the testing volume and measure to the nearest 10 mg/L or 5 mg/L, respectively. This requires a dilution that makes the color change more difficult to see so it may take a few tests to get used to.

Also, please note that molting problems are not just caused by an imbalance in minerals but also by diet and genetics. Finding your Ca:Mg ratio is a useful diagnostic tool to identify and fix a potential problem but this may not always solve molting problems.


  1. API Calcium Test
  2. 10-20 mL clear container that can be capped and shaken (link to cheapest 10 mL test tubes on Amazon)
  3. White background
  4. Good lighting


(Credit for modified API Calcium Test Kit procedure in freshwater goes to Planted Tank forum user Immortal1)

  1. Place 10mL of your tank water into the container. (Double volume for 5 mg/L accuracy)
  2. Add 20 drops of API Calcium Test Kit Bottle 1 to the container. (Double drop amount for 5 mg/L accuracy)
  3. Shake the container for 10 seconds.
  4. Shake Bottle 2 vigorously for 10 seconds.
  5. Add drops from Bottle 2 to the container until you see a change from pink to blue (keep going if purple). Be sure to count the number of drops added and shake well between drops.
  6. Multiply the number of drops by 10 to get the calcium concentration in mg/L or ppm. Multiply number of drops by 5 if Step 1 and 2 were doubled.
    • Example: 4 drops * 10 = 40 mg/L calcium

Step 2: Determining General Hardness (GH) of Freshwater Tanks

Follow the instructions of the API GH Test Kit (or whichever liquid GH test you prefer – do not use strips) to determine the GH of your tank.

Step 3: Determining Magnesium Concentration and Current Ca:Mg Ratio

Enter in your GH (select units) and calcium concentration from Step 1 to automatically calculate magnesium concentration and Ca:Mg ratio.

Calcium-To-Magnesium Ratio Calculator

Magnesium Calculator

Fixing Your Ca:Mg Ratio

If your Ca:Mg ratio is between 2-5:1, then don't worry too much about changing anything. However, if it is is significantly off the recommended 2:1, then it can be fixed relatively cheaply ($10) by adding in the required minerals. When adding minerals to your tank, always dissolve them in new water outside of the tank, then add slowly to avoid rapid water quality changes. Shrimp do not like rapid changes.

If your GH is high (>10 dGH) and your Ca:Mg balance is significantly off, then you may want to dilute your tap water with purified water (RO/DI from a home filter or local fish store, spring water from store [test GH/KH first to verify purity]) to lower the amount of whichever mineral is in excess before adjusting the ratio. This prevents an excessively high GH.

The following common household compounds can be used to increase mineral levels:

BE AWARE: If you choose to purchase products other than what is recommended below, be sure to check their purity and avoid any that may be scented.

Magnesium Supplement

Magnesium chloride (Most products are MgCl2 · 6H2O)

    • Magnesium is 11.8% of magnesium chloride products by weight.

Epsom Salt (Magnesium sulfate - MgSO4 · 7H2O)

    • Magnesium is 9.9% of Epsom salt by weight.

Calcium Supplement

Calcium Sulfate (Gypsum - CaSO4· 2H2O)

    • Calcium makes up 23.3% of gypsum by weight.

Calculation Explanation:

(Formulas are down below)

Let’s say you find that you have a GH of 7 dGH and a calcium concentration of 40 mg/L in a 10g (38L) tank. The calculator tells you that you have a Mg concentration of 6.1 mg/L and a Ca:Mg ratio of about 6.6:1. To achieve a 2:1 Ca:Mg ratio with your current calcium levels, you would need a magnesium concentration of 20 mg/L. This new magnesium level would put your GH at 10 dGH, which is within the recommended range.

You start by finding the difference between your current and desired magnesium levels (20 mg/L [desired value] - 6.1 mg/L [current value] =13.9 mg/L difference), then multiply by the tank volume to get the total amount of Mg required (13.9 mg/L * 38L = 528 mg of Mg required). MgCl2 is 11.8% magnesium so divide the amount of Mg required by 11.8% (528 mg/0.118% = 4.48 g MgCl2) to get the amount of magnesium chloride that should be dissolved into new water before adding it during the next water change.

You can also use this calculation to figure out the amount of magnesium chloride needed for future water changes to maintain the new levels.

Calculating How Much Calcium To Add To Your Shrimp Tank

If Ca:Mg ratio is below 2:1, then you may want to add calcium:

1. Calculate ideal (i) Ca concentration at 2:1 Ca:Mg ratio.

2. Calculate the difference (d) between Current Mg Levels (c) and i.

3. Multiply d by tank volume (v) (in liters – Converter) to get total Ca needed (t).

4. Divide t by gypsum Ca percentage (23.3%) to get total gypsum (G) needed.

Calculating How Much Magnesium To Add To Your Shrimp Tank

If Ca:Mg ratio is above 2:1, then you may want to add magnesium:

1. Calculate ideal (i) Mg concentration at 2:1 Ca:Mg ratio.

2. Calculate the difference (d) between Current Mg Levels (c) and i.

3. Multiply d by tank volume (v) (in liters – Converter) to get total Mg needed (t).

4. Divide t by Epsom salt magnesium percentage (9.9%) to get total Epsom Salt (G) needed. If using magnesium chloride, then use 11.8%.

Lastly, double check your expected GH with the addition of these new minerals to verify the calculations are correct and you aren't sending your GH sky-high. The equation to calculate GH in ppm is below. Divide the answer by 17.9 ppm to convert to dGH.


And that's all there is to adjusting your tap water - Isn't that easy??

Just kidding.

We tried to make this process as easy as possible but know it's not something everyone wants to go for. If you're on a budget and can't afford a filtration system for RO/DI water though, then we hope this helps!

Good luck and happy shrimpin'!


Pssst... Want to learn everything you need to know to have happy and healthy freshwater shrimp?  We've got you covered with clear, concise, and fun information in every lesson of The Shrimp School! 

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