Dec 16

Rocks for your aquarium

How to select the right rocks for your aquarium

While many aquarists around the world have no problem discovering their favourite varieties of fish, finding them, and then creating the perfect underwater environment for their fishkeeping hobby, determining how to select the right rocks for your aquarium can be a different story altogether. Many beginning aquarists are surprised to learn how important rocks can be in a marine environment.

Why are rocks important for your aquarium?

As you probably are already aware, your aquarium is essentially a miniature ecosystem that requires you to manage a precise chemical balance in which your fish can thrive. Thanks to water’s erosive qualities, the rocks in your aquarium will play a minor, but recognizable role in the “hardness” of your water— that is, the level of dissolved minerals in your water.

“Hard” water contains a higher level of dissolved minerals such as magnesium and calcium than “soft” water does. Obviously, the primary source of these dissolved minerals is the tap in your home, but the rocks that you introduce to your aquarium habitat can change the water hardness over time. Depending on the fish you wish to keep, this can be desirable or dangerous.

Additionally, well-placed and well-chosen rocks offer a beautiful decor that gives the tank a serene sense of beauty. Fish also love them, as the varied texture and landscape gives them lots of places in which they can hide and take shelter, just like their natural habitat would.

Aquascaping is enhanced with the addition of carefully selected rocks of various colours and textures. Make this choice based on the colours of the fish you plan to keep and whether the aquarium is to be planted or not.

Determining which rocks are safe

When it comes to finding out how to select the right rocks for your aquarium, its important to choose safe rocks, as certain types can be poisonous for your fish. There are several methods available to determining which rocks you can use:

• Purchasing aquarium-safe rocks. If you purchase aquarium-safe rocks from a quality pet store or aquarium supply centre, you can be relatively certain that the rocks will not gravely affect the hardness or pH level of your aquarium water.

If you choose to go this route, it is important that you purchase from trusted vendors, as some pet shops have been known to cut back on quality control and put unfit rocks up for sale.

• Testing outdoors rocks and gravel. Many aquarium enthusiasts and fish keepers like to take home interesting-looking rocks from riverbeds or other natural sources and introduce them into their aquariums. This approach requires testing, since outdoor rocks can contain high levels of calcium and other materials that will change the chemical content of your water and affect your fish. Granite, slate and sandstone are relatively inert and have little or no effect on the water chemistry. Also clay, although not strictly a rock, is a good source of rock-like material. Clay pots, pipes and slates can be used adding a nice brown colour to the landscape.

How to test outdoors rocks for aquarium use

If you have found some interesting rocks that you would like to introduce to your aquarium, there are two main ways to test them for use in your aquarium:

• The vinegar test. Vinegar reacts with calcium by fizzing and foaming on contact. If you pour a few drops of vinegar on your rocks and you see that they begin to react in this way, you should not use the rocks in your aquarium. This is an indicator of high levels of calcium. Rocks that do not react with vinegar can generally be used, but a more reliable test may be in order if you would like to be perfectly certain.

• The standing test. If you have some rocks or gravel that you would like to introduce to your aquarium and would like to test them securely, the best way is through the standing test. Let the rocks stand for a week in a bucket of the same water that you use for your aquarium, and then test the water hardness and pH level.

If you see that the water quality has not significantly changed, then you can reasonably expect that the rocks are aquarium-safe. Naturally, longer testing times will provide more detailed results, and help eliminate any doubt about the quality of the rocks or gravel you have found. When figuring out how to select the right rocks for your aquarium, the standing test represents the best way to be absolutely certain, although it takes time.

Also after adding new rocks it is wise to keep an eye on the fish over the following weeks to see if they show any sign of distress. Some rocks may very slowly release poisons into the water over the long term. If the fish do show some signs of distress, try removing the rock and do a 50% water change to see if the distress is relieved.

Freshwater vs. saltwater considerations

As you would expect, there is a marked difference between the types of rocks ideal for freshwater tanks and those that saltwater tanks can safely house. If you are a beginning aquarist determining how to select the right rocks for your aquarium, you need to base your choice of rocks on the type of water you are using.

While freshwater tanks are significantly simpler to manage, saltwater aquarists have additional concerns about maintaining the salinity of their tanks’ water. Given that some rocks can have poisonous effects, and that most will affect the water quality in some way over time, it is important to choose carefully and test your rocks.

An additional option that can help maintain excellent water quality, appropriate salinity, and balance a tanks’ pH level is live rock. Live rock is especially useful in saltwater tanks, but is also recommended for certain freshwater tanks such as the Malawi biotope, where it also helps create a decorative atmosphere in place of plants that may not be present.

What is live rock?

Live rock is a bit of a misnomer, since the material in question is neither a rock nor alive. Live rock is made up of pieces of coral skeleton that have broken off of reefs and are collected for use in home aquariums. These coral skeletons become natural biological filters, helping the nitrogen cycle take place effectively.

In this case, the material that you are introducing to your aquarium is designed to affect the water composition, but in a positive way. Live rock introduces helpful bacteria, algae, and tiny invertebrates that can improve the quality of your aquarium water. Live rocks can raise the salinity and the pH level of your tank water. If you are looking for attractive solutions on how to select the right rocks for your aquarium, live rock is an important element to consider.

As an added benefit to saltwater aquarists, live rock can form the foundation of bright and colourful coral colonies that distinguish saltwater aquariums from their freshwater cousins. Many ambitious saltwater aquarists choose these rocks for their aquariums specifically for those species of bright coral to grow.

Additional considerations for your aquarium rocks: gravel

Since gravel often forms a significant element of any aquarium’s substrate base, it should be given special attention due to the additional concerns over its small size and numerous individual particles. Gravel offers a very natural appearance for your tank. The colour chosen must blend in naturally or pleasantly contrast the rock work. Examples are grey rock work with yellow sand or salmon pink rockwork with grey gravel.

Large-grained gravel allows waste to penetrate the substrate and stick unpleasantly to the bottom of the tank. This, in turn, will affect the water quality and the health and lifespan of your fish. For this reason, many aquarists prefer to use small-grained gravel or even sand. If you insist on using large-grained gravel, you will have to carefully and efficiently clean your tank regularly in order to maintain ideal water conditions.


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