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Calcium Block for Turtles

Turtles and other aquatic animals need calcium.  A turtle without sufficient calcium is susceptible to soft shell disease and a range of other maladies.    In nature calcium is obtained from diet, but in a tank or other artificial environment where food is perhaps not the ideal natural balance, calcium can be provided by dietary supplements or from a slow release calcium block. 

Calcium blocks can also be a useful pH modifier as the calcium reacts with acidic animal wastes to form benign soluble calcium salts and carbon dioxide.

Chances are if you are reading this then you are already convinced about the potential benefits of calcium.



Figure 1.  Yurtle Waiting for New Calcium Block


Yurtle used to get a calcium block every month.  He would nibble at it from time to time but otherwise it simply dissolved in the tank.  I’d simply purchase a new fish tank calcium block from the super market every month for a few dollars.

A question that you might be asking is why will turtles take bytes out of a calcium block?  This stuff doesn’t smell like food (or anything much after all).  I figure that they instinctively know when they need calcium and nibble at a block accordingly.  The behaviour is similar to that of horses and cows with salt licks.

In recent times our local supermarket has stopped stocking fish tank calcium blocks as the manufacturer has apparently closed down.  I searched further afield and found some other  calcium blocks with all manner of ‘essential additives’ but these were rather expensive at over $12 each.

So, for Yurtle’s heath, and my budget it was time to try and make my own calcium blocks.

You can find some basic recipes on the interweb.  The fundamental constituents are calcium carbonate (chalk) and calcium sulphate hydrate (plaster of Paris or gypsum plaster).  The proportions are about 50:50 by volume.  Simply mix the two powdered constituents and add sufficient water to form a thick slurry.  Let it stand for a minute to ensure that the calcium sulphate is fully hydrated, adding more water if necessary.  Then pour the mixture into a mould or onto a sheet as a biscuit, and leave it to set.

The mix should solidify within an hour and this is a good time to remove it from a mould.  Air dry at above 10C (18C is better) for a few days and you’ll have a calcium block.

Powdered calcium carbonate is readily available from brewers’ supply shops and plaster of Paris can be obtained from hardware outlets.  Both are relatively low cost chemicals that are non-toxic (they are often used in the formulation of anti-acids).

While some forms of gypsum wallboard plaster may be a useful source of calcium sulphate hydrate they can contain modifiers such as clay, lime, polyvinyl alcohol, mica, and cellulose.  Avoid anything that has reinforcing mineral fibres or anti-mould properties.

Both constituents have a low solubility in water, increasing with increasing temperature. The solubility of calcium carbonate in pure water is about 15 mg/l but this increases in natural waters due to its reaction with dissolved carbon dioxide to form soluble calcium bicarbonate.

The solubility of calcium sulphate is about 2.5 g/l.

As the calcium constituents dissolve, the water becomes increasingly hard.  This is typical of mineral waters and is not detrimental to the aquarium environment.

Before mixing up my first batch of turtle calcium I made a 3D printed mould in the shape of a stylised turtle.  Any  mould must have plenty of draft to ensure that the set mixture will release easily.  The print was sanded and solvent smoothed to assist with release and I applied some olive oil to as a release agent although fish oil would probably be more to Yurtle’s liking.

An oval or hemispherical shape is desirable because it provides a relatively low surface area to volume ratio that should slow the rate at which the block dissolves in the water.



Figure 2.  141 g Calcium Block Mould


There is considerable scope for putting other additives into the mix.  I haven’t tried this yet but ground up dried turtle food might be a good option.

My initial batch comprised six tablespoons of Calcium Carbonate and six of  hydrated Calcium Sulphate with enough tap water to form a slurry with a consistency of treacle (just pourable with patience).

    Over time I have adjusted the mixture ratio to 1:3 calcium carbonate to calcium sulphate.  This mixture lasts much longer in the tank without falling apart.

With the mixing completed I simply poured/scooped the compound into the mould and put the excess onto some grease proof baking paper.  Then I waited about an hour until the mixture was firm to the touch and knocked the mould out.  Both blocks were air dried over the next 24 hours but they need several days to thoroughly dry.

The turtle shape weighed 141 g and the biscuit 81 g.  The density of the set mixture is about 1.35 g/cm3.  I had clearly mixed up too much.



Figure 3.  Turtle Calcium Block (141 g)



Figure 4.  Half of Calcium Biscuit (31 g)


I have also made a scaled down version of the turtle mould for a 50 g block, splitting the mould to assist with release.  This will need just 1 1/2  heaped tablespoons of Calcium Sulphate and 1/2 a tablespoon of Calcium Carbonate.



Figure 5.  50 g Split Mould for Calcium Block


Once dried the blocks had a soft cool feel similar to talcum powder.  They were quite solid and chipped with moderate impact. The biscuit broke cleanly just like a regular biscuit that you might have with your morning coffee.

What do the blocks taste like?

If you nibble a bit of your calcium block it should have no taste at all and a powdery sensation.  Rest assured that I have tried this with no adverse heath effects – and I have no intention of poisoning Yurtle!

In the early days of chemistry it was a common practise to taste stuff (hey - we eat chemicals every day).  I have several rather ancient chemistry books where the taste of some rather nasty compounds are described.  Not surprisingly many of these early researchers died well before their natural expiratory date.

Having completed my taste test I was quite confident that the blocks wouldn’t harm Yurtle.  So I place half of the biscuit (50 g) into Yurtle’s tank.  This is a large tank containing about 1,000 l of water at 27C. The tank has a constant stream of air bubbles (so carbon dioxide saturation can be assumed).

As an aside 1,000 litres of water weighs about a tonne and the glass is a four man lift. This weight is distributed over an area of 1.125 square metres.  Some years back when Yurtle’s tank was in a rental property the floor joists deflected appreciably (perhaps as much as 20 mm over a metre span) over time, even though the tank was supported on three joists right beside the supporting wall.  When I installed Yurtle’s tank here at home I put additional piles under the supporting floor joists to make sure that they didn’t deflect under the load.  In more recent times we have had a significant 7.8 Richter Magnitude earthquake.  Aside from the Tusnami in the tank, nothing moved.  Other folk were less fortunate.  I know of at least one fish tank that toppled and broke and the tank occupants did not survive the event.

The block immediately settled to the bottom of the tank and initially gave off a steady stream of bubbles (just like commercial products) due to air escaping from pores in the surface of the block.

Within 10 minutes Yurtle was investigating the block and after 30 minutes he had nibbled at it and clawed at the surface (typical Yurtle behaviour with commercial blocks).



Figure 6.  Calcium Half Biscuit in Tank
(already partially eaten by Yurtle)


In order to investigate the life of a block I placed the remaining 31 g half biscuit in a litre of tap water to observed its rate of decay/decomposition without Yurtle’s assistance.  I’ll report back on this after a few weeks.  If the block crumbles or dissolves too quickly I’ll need to revise the recipe.



Figure 7.  Day Zero of Rate of Decomposition Test


After 2 weeks the test calcium block was largely intact and solid, with just a small amount of powder having fallen and distributing over the base of the vessel.  However Yurtle’s block had almost entirely disappeared, due to being eaten.  So the basic recipe looks okay.



Figure 8.  Two Weeks of Decomposition Test



Figure 9.  After 15 days Hardly any Block Left in Yurtle’s Tank


The cost of materials for home made 50 g blocks is about 73 cents and they take just a few minutes to make.  A kilogram of each constituent will provide Yurtle with calcium for years.


The only hassle with the current process is removing a set block from the mould in one piece.  I have reprinted my 50 g turtle block mould using NinjaFlex which is a very flexible and strong rubber 3D printer filament.  The blocks come out of the mould perfectly and there is no need for a release agent (like fish oil).  You can read about 3D printing with NinjaFlex by clicking on the link.


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