Photo by Rafal Ciesielczuk on Unsplash.

Do Fish Need Rocks: Pros and Cons of Lava Rock

Rocks come in many shapes, texture, and colors. Stacking them carefully or gluing them together can be the basis of stunning aquascapes or peaceful slices of underwater nature.

But fish do not need rocks in their tank. All the same, rocks provide a number of benefits. They are natural hiding places for fish, regulate the water temperature, and shape up the overall aesthetic of the tank. Rocks can create caves and other hardscape for fish to hide or lay eggs in.

What’s more, rocks slowly absorb heat from the water, keeping the temperature in a tank more stable. If, for whatever reason, the temperature rises sharply, rocks will slow the process down by sucking up heat. Respectively, if the heater breaks, the heat stored in rocks would offset the cooling of the tank.

Most importantly, rocks build the very aesthetic of a tank. They can be used for a variety of different looks, from a natural tank to a minimalistic bare bottom setup.

However, it is important to choose the right type of rocks for your tank. Some stones can leach chemicals into the water harming your fish.

In the article, I speak mostly about porous volcanic rocks because they are light, come in many sizes, and I like how they look. They have other benefits, too, that I discuss below. But pretty much anything that can be done with volcanic rocks is possible with any other kind of safe stone or rock.

Benefits of Volcanic Rock in Fish Tanks

Most fish species will do just fine without any rocks or other substrate in their tanks. Bare bottom tanks have many advantages, but they are not for everybody. Some bottom-dwelling species like loaches and corydoras would do much better with a sandy bottom and hiding places.

Rocks provide places to hide and dwell, and surfaces for bacteria that maintain the water chemistry intact. But that’s not all.

Let’s break things down a bit.

Rocks Provide Hiding Places for Fish

Volcanic rocks are excellent for making your tank interesting to look at but also interesting for the fish. While it is true that you can play with your fish, a better, more sustainable way to keep them happy and healthy is to make their environment suitable for their needs.

Rocks break the monotony of the bare bottom tanks and provide a place for fish to explore and hide. Now, you don’t have to go over the top and fill the aquarium with rocks. You can still maintain a minimalistic look if that’s what you want, adding a volcanic rock or two, just to accentuate certain places of the tank.

What’s more, a rock can anchor plants, especially species like anubias that like their rhizome outside the substrate.

Volcanic Rock Helps Beneficial Bacteria Grow

Volcanic rock has an uneven surface that is excellent for beneficial bacteria to colonize and inhabit. These bacteria feed upon harmful chemical substances that foul the water column. These invisible organisms are chemotrophs, i.e. they eat chemical components.

To be precise, they break down ammonia and nitrite. Both are harmful to fish. There are various kinds of beneficial bacteria, but they share common reproduction patterns. They take about 24 hours to grow in numbers and need surfaces to live upon. Filter media, substrates, plants, and rocks are the perfect places for them to colonize.

Volcanic rocks are better than most because their unevenness increases their surface area manifold. The holes and crevices can house many more bacteria than a regular river stone of roughly the same proportions.

Volcanic Rock Is Light

Alongside the bacteria benefits, volcanic rock has another major advantage over regular stones. They are lightweight and that can make a difference if you use a lot of stones. The density and weight of river stones and normal rocks can become too heavy for your floors or aquarium stand.

Now, there are lava rocks that are as heavy as any regular stone, but porous volcanic rocks sold in fish stores are considerably lighter than regular rocks.

Rocks Maintain Aquarium Temperature

That’s right, rocks and other large hardscape elements regulate the water temperature. They absorb heat and release it slowly. More slowly than water, at least.

Should the ambient temperature rise sharply, the rocks will keep the water column a bit cooler. Essentially, they are large bodies that are hard to heat up and will absorb heat from the water.

Conversely, if your aquarium heater breaks and the ambient temperature drops, the heat stored in rocks will start dissipating in the water column, slowing down the cooling period.

How much rocks will affect the temperature depends on their size and number. A few medium-sized rocks will have a greater impact than one large stone.

As a side note, if you want to keep the temperature higher without a heater, consider turning off any mechanical filter or reducing its strength. Water movements — currents and waves — cool things off more rapidly.

Volcanic Rocks Are Pretty

Volcanic rocks are beautiful and are great for aquascaping. Their irregular shapes often provide ground for interesting structures and easy stacking.

Of course, beauty is subjective, but there are many types of rocks out there. Different textures, colors, sizes, and overall feel to make every fish tank interesting and unique. And most rocks are free, which is a nice boon. Even volcanic rocks are if you live close to a volcano.

Of course, beauty is subjective, but there are many types of rocks out there. Different textures, colors, sizes, and overall feel to make every fish tank interesting and unique. And most rocks are free, which is a nice boon. Even volcanic rocks are if you live close to a volcano.

Overall, rocks have many benefits. Their pros are infinitely more than their cons. 

Cons and Dangers of Rocks

Volcanic rocks are safe, especially after proper cleaning, but there are stones that should be avoided. But volcanic rocks can pose a potential drawback, namely, price. They are quite affordable in most fish stores but aren’t free.

Another potential issue is the possibility of breaking the aquarium if you drop a rock on the glass or if a stone structure collapses. This may sound more like a theoretical risk but work with rocks long enough and it will happen.

Now, I mentioned rocks are free. Be careful where you collect them from. Generally, river stones and rocks are fine, unless the water source is very polluted. Even then, with proper treatment — boiling, hard scrubbing, using vinegar — they can be made safe for aquariums.

Honestly, every single rock you plan to use should undergo at least boiling and scrubbing before putting it among your livestock. Even those bought in the fish store.

But there are stones that can raise the pH of your tank. That can be a desired effect as guppies, swordtails, and many cichlids prefer base water. The type of rocks that alter pH contain calcium carbonate. Typically, they are sedimentary rocks like limestone, sandstone, and marble.

Again, they aren’t bad per se but should be used sparingly and with the type of fish that prefer higher pH, assuming your water source is on the acidic side.

Another thing to look for is sharp protrusions and edges that can damage fish easily.

Other than that, rocks rock hard!

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