What Causes Low Calcium and Magnesium in a Fish Only Aquarium?

Whether you manage a freshwater or saltwater aquarium, it is imperative to maintain a clean, safe and healthy environment for your aquatic species.

The water chemistry in your aquarium has a critical impact on the health of your fish. Depending on the type of fish tank, your aquarium will either have saltwater or freshwater.

In fish-only aquariums, reef tanks, and fish-only with live rock tanks, saltwater is used; while planted, cichlid, predator, biotope, and brackish tanks make use of freshwater.

When it comes to saltwater aquariums, such as a fish-only tank, you need extra equipment and conduct tests on a regular basis to maintain the right water chemistry.

This makes saltwater fish-only tanks more difficult and expensive to manage.

The two elements crucial to maintaining a healthy environment in fish-only aquariums are calcium and magnesium.

Regular testing of aquarium water is necessary to monitor their levels. Without this assessment, calcium and magnesium levels can drop precipitously.

But what exactly lowers the calcium and magnesium levels in a fish-only aquarium? Let’s find out:

What Lowers Calcium and Magnesium levels in Fish Only Aquarium?

Before we go into the calcium and magnesium levels, it is important to understand what a fish-only tank is. A fish-only aquarium is the most basic form of a saltwater tank that doesn’t house live corals and other motile or sessile invertebrates, nor any live rock.

The only thing you need to worry about is the fish.

But does this mean that a fish-only aquarium hosts only fish? No. Instead of live corals and rocks, decorative type rocks, seashells, synthetic or natural dried pieces of coral artificial plants, and similar objects are used.

The reason why you want to eliminate live rocks and corals is that when you have them in your fish tank, you need to put in extra effort and investment to meet their requirements as well.

For instance, corals might require you to install additional equipment for their needs, while live rocks have extra cultivation needs that need to be met.

But most importantly, all live corals demand large amounts of calcium to develop their skeletons. Thus, reef tanks that host live corals require you to maintain high calcium levels.

Likewise, corals also make use of magnesium for the growth of their calcium carbonate skeletons. Magnesium, however, is also used by coralline algae for their calcium carbonate deposits.

Hence, aquariums carrying live corals and coralline algae require high levels of both calcium and magnesium.

Fish-only tanks, on the other hand, have no live corals or coralline algae. Then what lowers calcium and magnesium levels in them?

To begin with, fish-only tanks do not have a very large demand for either of the calcium or magnesium.

Fish only aquariums do comprise of some algae which consume small amounts of both calcium and magnesium. This leads to a gradual reduction in the calcium and magnesium levels.

The only way you ever have to worry about calcium and magnesium levels in a fish only tank is an overabundance of algae growth. The regular water changes should efficiently maintain high levels of calcium and magnesium.

There’s one additional thing that explains the scenario. What fits into this equation is the strong correlation between calcium and alkalinity levels.

If alkalinity levels get too high, calcium levels fall dramatically and precipitate out of the solution in the form of insoluble calcium carbonate.

Ideally, alkalinity levels should be maintained between 2.5-3.5 meq/L to maintain calcium levels between 400-500 ppm. Hence, make sure you monitor both alkalinity and calcium levels every week and supplement accordingly to maintain the desired levels.

If you ever faced a problem with calcium and magnesium levels in your fish only tank, a fresh batch of the salt mix will replenish the levels for you.

This means that managing a fish-only tank is less expensive, but you can’t say whether it’s the easiest to manage. You need to maintain a nitrogen cycle in any type of aquarium.

This process involves the breakdown of toxins such as ammonia into less harmful substances. What helps in this breakdown is the beneficial bacteria that fish-only tanks lack.

Reef or FOWLER (Fish Only with Live Rock) tanks have large colonies of beneficial bacteria that speed up the nitrogen cycle.

Therefore, fish only tanks require more frequent maintenance. Also, without live rock, you will more often have to make water changes to maintain high-quality water in your fish only tank.

Hence, you certainly need a test kit to test the quantities of ammonia, nitrate, pH levels, and nitrite; elements that determine water quality.

How to Test Calcium and Magnesium levels?

Testing the calcium and magnesium levels forms a critical part of your regular aquarium maintenance. You can easily find test kits for testing both elements at a nearby pet shop or on an online store.

A typical testing kit would include a reagent, a color chart, and a vial. All brands generally follow the same procedure with minor variations in product specifics.

Read the instructions given on along the kit. To give you an idea of the procedure, here is what you will need to do:

Begin by collecting a sample of your aquarium water in the vial. Then, take the reagent bottle and add the prescribed number of drops to the sample. While you add the drops, hold the reagent straight upside down. Put back the cap on the vial and shake it so that the water mixes with the reagent.

See the instructions on the kit to determine whether you need to add another dose of reagent and how long you should wait after mixing.

Once you’ve waited for the prescribed length of time, use the color chart to compare the color of the sample. This will reveal the calcium and magnesium levels in your aquarium.

The general unit to measure calcium is parts per million (ppm). Your aim should be to maintain calcium levels close to those of natural seawater, which is around 420 ppm.

In the case of reef tanks, you will need to maintain calcium levels between 380-450 ppm for the coral and invertebrates to thrive well.

In such aquariums, the chances of calcium levels getting too low are high. You will hardly face a situation when the calcium levels get too high.

If you’re a fish-only aquarium owner, you won’t have to worry about calcium levels, but if you’re reef aquarium enthusiast, you can increase calcium levels in many ways.

For instance, you can use calcium-rich decorations such as tuffa rock and coral sand, add liquid supplements such as kalkwasser drips, or use active equipment such as calcium reactors.

Similarly, maintaining magnesium levels is also important for reef tanks. Corals and other invertebrates use it for metabolizing calcium. Furthermore, its interaction with tank water helps dissolve calcium in the water column.

Magnesium levels are also measured in terms of parts per million (ppm). Again, your aim is to maintain it as close to the natural water as possible, that is, somewhere around 1285 ppm.

You need to maintain magnesium levels from 1250 to 1350, but you do have a leverage of 50 ppm both in the upper and the lower range.

This means that levels beyond 1200-1400 can create issues. You can purchase liquid supplements at local pet shops that help maintain ideal magnesium levels in your fish tank.

Other Testing Guidelines

There is no single set of rules defining how often you should test your aquarium water chemistry. Different experts come up with different guidelines.

Regarding how often you should test the aquarium water, Reefkeeping Magazine suggests that you conduct the tests occasionally, which is often interpreted as ‘quarterly’.

On the other hand, Fish International magazine recommends that aquarium water be tested on a monthly basis.

However, don’t strictly stick to these intervals. You should consider testing the water as soon as you spot any signs of low magnesium or calcium.

For instance, if you see withdrawn tentacles or unusually slow growth in coral, you should immediately know that calcium levels must have gone low.

But suppose you tested the calcium levels and they turned out to be normal. Magnesium levels must have gone deficient.

Besides, another indication of low magnesium levels is that the calcium won’t effectively dissolve in the water and fall out of the solution. If you sense anything like that, test the magnesium levels in the water.

Closing Thoughts

To sum it all up, you shouldn’t face any issues with the calcium and magnesium levels if you own a fish-only aquarium.

The real challenge with maintaining calcium and magnesium is associated with managing reef aquariums that are rich in coral and other invertebrates with hard exoskeletons and shells.

The underlying reason is that they absorb large amounts of magnesium and calcium to grow and thrive. Thus, regular testing is extremely important for reef aquarium owners.

Fish-only aquariums are only advised to keep an eye on algae growth, excessive amounts of which can cause calcium and magnesium deficiencies.

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