Frequently asked questions on calcium dosing


Matt Clarke answers some of the most frequently-asked questions on adding calcium to the reef tank.

Why is calcium important?

Calcium is required by coralline algae, invertebrates and corals for growth. The calcium level on most reef environments is around 400-420 ppm. You should aim to keep calcium at this level if you want to keep corals, clams and other invertebrates growing healthily.

While marine fishes are adapted to live at this calcium level, they show few problems when kept at levels slightly higher or lower than normal.

Won't my salt provide all the calcium I need?

Some of the older salt mixes on the market are designed for use with hard, alkaline tapwater, rather than the RO water most modern reefkeepers now use. This means that they often produce water with a low mineral content, so alkalinity, calcium and magnesium levels could all be lower than they should be. Some salts are well-suited to use with RO and can make water with mineral levels that closely resemble or even exceed those of natural seawater.

In our tests of salt mixes Instant Ocean, Reef Crystals and Kent Marine made the best water when mixed with RO.

What water conditions should I aim for to get good coral growth?

Aim to provide water that resembles the chemistry of natural seawater: a calcium level of 400-420 ppm; alkalinity 7-10 KH (2.5-3.5 meq/l); magnesium 1200-1300 ppm, pH 8.2-8.2 and a salinity of 35-36 ppt. However, many experts now reckon it's beneficial to have a higher calcium level (up to 500 ppm) for improved coral growth, although getting the calcium level this high can cause mineral imbalances elsewhere in the tank, particularly decreases in alkalinity, if done incorrectly.

How can I work out the calcium demand of my tank?

If you have lots of hard corals, the calcium will get depleted more quickly than it would if you kept predominantly soft corals. You can get a rough idea of the calcium demand by doing a simple "before and after" calcium test on your tank for a week or so. Your calcium dosing regime needs to replace at least as much calcium as is removed by your tank's calcium demand.

How can I add more calcium?

If the tank only has a low calcium demand, then water changes can help to boost calcium to natural levels. However, continuous dosing is often required if you have lots of corals or other organisms taking-up calcium. There are four main ways to do this: calcium chloride additives, two-part solutions, kalkwasser or calcium reactors.

> Calcium chloride additives

Most of the liquid additives on the market contain calcuium chloride. This increases calcium levels, but over time, can lead to a drop in alkalinity. Calcium chloride-based additives are best for tanks with a low calcium demand and although easy they can be costly to use.

> Kalkwasser

Kalkwasser is a highly caustic liquid made by adding calcium hydroxide powder to RO water. When mixed, a clear portion forms which needs to be added to the tank slowly to replace evaporational losses. When used correctly kalkwasser can increase calcium and alkalinity. But, when used incorrectly, it can cause a massive increase in pH and a drop in alkalinity. It's often hard to get the levels over those seen in natural seawater.

> Two-part solutions

Two-part solutions, like Two Little Fishies' C-Balance, can boost calcium levels and alkalinity in a balanced ratio without skewing the chemistry of the water. These are your best bet if you can't afford a calcium reactor.

> Calcium reactor

Calcium reactors can control calcium, alkalinity and magnesium levels and add trace elements with few negative effects. By pumping carbon dioxide gas into the reactor, which sits alongside a sump under the tank, the aragonite media inside the reactor dissolves much more quickly and releases lots of minerals.

Although expensive to purchase initially, they can supply large amounts of calcium and can actually be much more cost effective to use than additives. Based on our tests on calcium additives last year, if you have a big calcium demand on your tank and you boost calcium levels using bottled supplements, a reactor could pay for itself within a year.

To determine whether a reactor is the most cost effective solution for your aquarium, enter your details into our calcium cost calculator.

How often should I add my calcium additive?

Although most manufacturers will tell you to add their product to your tank every two days, or whatever, it's not really very wise to add anything without testing the calcium, alkalinity, magnesium and pH levels first. You may not need to dose at all if there is a very low calcium demand. Or, you may need to exceed the stated dose.

Few manufacturers disclose the concentration of their products, so simply following their instructions on dosing often won't tell you how much calcium a dose will replace. In our tests, some provided very little calcium at all so may only be suitable for supplying low calcium demands. This lack of concentration means that some of them can be very expensive to use, and in many cases you could be much better off going for a calcium reactor, even though it will cost a lot to buy...

The Buyer's guide to calcium additives is archived in the buyer's guide section of the PFK website.