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Why the need for Calcium & Magnesium

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I thought I'd add this topic to accompany Blue Bolts great thread on correct ratios and dosing of Calcium and Magnesium.

So why is Calcium & Magnesium important to both aquarist who keep fish and those of us who keep shrimp?


CaSO4 Calcium sulfate in your aquarium will keep a more stable electrolyte balance (for osmotic function), while magnesium is another important element that works with calcium.

A proper amount of Calcium and Magnesium in your aquarium will affect your shrimp or fish health positively, including fish from low pH environments such as Apistos, Discus or German Rams

Magnesium and Calcium have been shown to increase resistance to degenerate diseases by lowering the acidity in the body. This will help with prevention of ich and fungus in your fish.

Calcium also helps in healing and stress, and without proper calcium levels healing may be difficult or impossible. Calcium is also important and has been shown to both prevent and treat Hole in the Head disease common to cichlids (also referred to as HITH).

The addition of antibiotics (such as Tetracycline) will lower calcium absorption, while the presence of correct amounts of calcium in the aquarium water will considerable reduce the toxic side effects of Malachite Green which is why a GH (for freshwater calcium measurement) of 100 ppm (for freshwater) is SO VERY important to ich treatment.

Calcium can adversely affect the kH of a discus aquarium when combined with sodium carbonates or bi carbonates, which is generally kept at a pH below 6.5. Not that calcium has a direct impact on raising or lowering kH, but that it assists in buffering the water to avoid swings in kH and thus pH.

I have successfully used sources of calcium in discus (low pH) aquariums by using a mix of RO (Reverse Osmosis) water and tap water (dilution will vary depending on your tap and tank water parameters). Then add electrolytes/mineralsto the RO water and add peat to the filters. This method has been used successfully with discus and calcium added with no pH climb.

The peat will leach minerals into the water that will bond with the Carbonate thereby preventing it from bonding to the “H+†ion, thereby lowering KH and subsequently pH. This is a good method for planted tanks as the nutrients can also be used by plants.

Calcium sulfate is soluble in water. However, it's solubility is extremely poor. Only a small amount will dissolve, and this will take place extremely slowly over time. To improve solubility, use a mortar & pestle to crush into a fine powder before adding to water.

Calcium is particularly important to the average shrimp and invertebrate keeper.

Calcium plays a huge role in Osmoregulation. And as such plays a big role in assisting the shrimp's moulting cycle.

Ever have shrimp die during a failed moult? Check the Calcium levels in your water.


Magnesium is important for proper osmotic functions in fish and invertebrates. Magnesium is essential for Calcium assimilation, so when magnesium levels are low, the calcium supply becomes exhausted. For this reason, Magnesium is better added in the proper balance with calcium (which both are essential to each other for proper utilisation).

Epsom salts that contain magnesium sulfate, are best used for therapeutic reasons such as to aid in flushing the system as it aids in and speeds osmotic function, and helps to move fluids out of the body.

Sulfates, one of the major components of Epsom Salt, have been shown effective in improving nutrient absorption and toxin elimination.

Magnesium, the other major component of Epsom Salt, plays a role in the activity of many enzymes.

Also note that Epsom salts (MgSO4) do NOT evaporate or decompose, so only add more after water changes.

Where to Buy:

Calcium Sulfate CaSO4 can be purchase from

Magnesium Sulfate can be purchased from

  • Aquotix online store (
  • Bunnings (Manutec Epsom Salt)
  • Big W (Manutec Epsom Salt)
  • Ace Chem ( - bulk orders

If there are more sources, let me know and I'll update the list.

Edited by jayc
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Thanks for the follow up info jayc....EXCELLENT. :encouragement:

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Thanks BB.

You're still the master.

I knew the benefits. But finding the correct dosage was the key you provided.

It was like having the medicine to an illness but not knowing the dosage to take.

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I see a few things wrong with this, the formula for Calcium Carbonate is CaCO3. Calcium only affects KH if it is added as CaCO3, not as Gypsum or Calcium Sulfate as is the formula you have written.

Sodium Carbonate or Potassium Carbonate will by themselves affect KH, Calcium has nothing to do with it.

Peat works by removing calcium from the water column through ion exchange, although it will also dissolve Calcium bearing minerals due to chelation from humic and fulvic acids.

Calcium supply is constant, Magnesium just controls uptake by organisms. Without Magnesium, there is still plenty of Calcium, it is just unusable.

It's also worth explaining that GH is made up of both Calcium and Magnesium. It is the weight of CaCO3 that is the molar equivalent of all 2+ valence ions, so to increase GH you aren't relying solely on CaCO3.

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Thanks for picking up that mistake. Indeed CaSO4 is indeed Calcium Sulfate. Fixed.

In regards to that sentence about Calcium / KH / carbonates ... I'll need to re-word it.

I didn't mean that calcium has a direct impact on raising KH ... rather, it is more an aid in maintenance of these parameters by neutralizing acids that result from organic decomposition.

I knew I shouldn't have rushed that section. I guess what I was trying to say there was something like ...

Too much calcium can make it difficult to reduce pH when keeping fish or invertebrates that prefer softwater.

Sentencing structure of that paragraph welcomed.

Thanks for the other notes too.

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Very cool write up dude, like you said, know we have both the reason & ratio for calcium & magnesium, thanks for sharing :victorious:

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Added a section on "Where to buy"

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Nice job JC and an excellent learning tool for people new to not only shrimp keeping but also fish. Especially those keeping livestock at low ph levels.

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