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Rocks are a vital part of our aquarium, so we'll discuss that first. Then, we'll learn about the different types of rocks available and, finally, we'll discuss my choice and how to set them up.

A Home for Bacteria

The rocks we put in the tank will provide surface area for colonies of beneficial bacteria that are vital to the tank. We cannot have any other living things in the tank until these bacteria are established in sufficient quantities and we can confirm that they are doing their job. The process of establishing these bacteria is called cycling the tank and we'll talk about it in more detail later: just know that it is a crucial step and the rocks are the key. You can also add some artificial media to provide more surface area but, in most cases, the rocks will suffice.

A Safe Place for Critters

Rocks also provide hiding places for fish and any other critter that wants to feel safe. It's important to arrange your rocks so that fish can find some nooks and crannies to hide in. Or, at least some "corridors" they can swim through to escape aggression. Almost all fish like to retreat to a cave when they feel threatened, or when it's time to go to sleep. Fish also use the rocks as a reference point to establish territories - moving the rocks disrupts them and they have to start over.

A Foundation for Corals

When we add corals, most will be glued to the rocks - so we need a variety of places to do so. We'll need the ability to move corals higher and lower in the tank so we can get them closer or farther from the lights. So, we need our rocks to be arranged in a way that accommodates this - with some high peaks and some low plateaus.

Anemones like to hide their foot in a crevice and stretch out to get light, so we need to have crevices for them although they're not too picky.

Types of Rocks

Not all rocks can be placed in reef tanks, so we need to look for rocks that are specifically for this purpose. A lot of types of rocks that are commonly used in freshwater aquariums are not suited to our reef as they may leach unwanted compounds into the water. It is also not a good idea to use any rock you find outside or get from a beach or the ocean. These rocks could contain any number of pollutants or undesirable life that will just cause us headaches later on.

Given that, we are stuck with buying rocks. There are two main types of rocks we can buy: man-made, and natural.

Man-made Rocks

These rocks are made with ingredients that are known to be safe for our reef tank. They are also colored, so they look a bit more attractive right from the start. Because they are made, they come in interesting shapes that are not common in nature but may look good to you. The leading maker of rocks is Real Reef Solutions.

Personally, I'm not fond of the look of these rocks. I'd rather use natural rocks.

Natural Rocks

Natural rocks are ones that were in the ocean many, many, many years ago. Now, the oceans have receded and these rocks ended up buried in the ground. Enterprising companies mine these rocks, clean them, cut them and sell them to us. These rocks are safe and are generally lighter colored. The leading company is Marco Rocks. They cut some rocks to have a flat bottom and call them "foundation" rocks. Some are cut so that they have a flat bottom and top and are called "pedestal" while others are left as they are.

I chose Marco Rocks for this tank.

Live and Dry Rock

As soon as you start researching rocks for the tank, you'll run into these two terms, so they're worth explaining.

Live rock is rock that has been sitting in saltwater for a good while in giant commercial vats, at your local fish store or in a friend's tank. Live rock has living things in it which is both a good thing and a bad thing. It's good because it gives you a head start establishing colonies of beneficial bacteria and gives your tank good biodiversity from the beginning. It's bad because some of these living things could be unwanted pests. I prefer to start with dry rock for this reason - to avoid nasty critters from getting into the tank right away. It is possible to take live rock and cure it to kill all the living things in it. At that point, it becomes dry rock.

Dry rocks are literally dry and should have no living things in them. This guarantees that no pests are introduced but takes a bit more time to cycle. The Marco Rocks I chose for this tank are dry.


Buying Rocks

Although you can buy rocks online, I prefer to visit a local fish store. When you buy online, they may be expensive to ship and could get damaged in transit. Also, you don't know what you're going to get and may end up not using a large part of the shipment.

I'd rather go to the fish store, sit on the floor and start pulling out rocks from their bin, arranging them and picking out the ones I like. Most fish stores are very understanding of this behavior and, as long as you're not blocking the aisle, will let you carry on undisturbed.

I brought a piece of cardboard that is the same dimensions as the tank to the store, so that I could more easily visualize how the rocks will fit. I visited two stores to get all the rocks I wanted.

How Many Rocks

Rocks are sold by the pound and a very rough estimate is to get 1 pound of rocks for every gallon of water in your tank. It all depends on how you will arrange them. I tend to buy a bit less than that and see how it goes: if I need more, I can go back to the store.

I ended up with 6 rocks that weighed 18 lbs in total. At $3 per pound, that came out to $54. At the risk of getting ahead of ourselves, I am tracking all the expenses.


This is the process of arranging the rocks so that we accomplish these main goals: a pleasing look, lots of caves for fish to hide and lots of places for corals to grow. This can take some time but is also a fun part of the tank setup.

Cut out a piece of cardboard that has the same dimensions as the display section of the tank and place it on a table or the floor. Alternatively, you can use masking tape to outline the dimensions of the tank. Then, take out your rocks and start arranging them until you've got something you like.

Keep the following things in mind:

  • Rocks should be at least 2 inches from the inside of the glass so we can clean it.

  • They should be stable and difficult to knock over: the last thing we want is a falling rock cracking the bottom of the tank. In some cases, you may need to glue them together to prevent this from happening. There are many reef-safe products you can use for that.

  • Leave some of the bottom uncovered, so we have an open area for creatures that enjoy a bit of sand.

  • Ensure that your structures are not too tall and end up jutting out of the tank, interfering with the lid.

Once you're done, take a couple of pictures of the end result as it can be very difficult to recreate it inside the tank. This is what I came up with:

Give the rocks a quick rinse with regular tap water to wash off the dust and let them dry overnight.


Now, carefully place the rocks inside the tank, directly on the glass. Don't put anything at all under them. It is better to put the rocks in before the sand because some creatures like to dig up the sand and, if the rocks are on top of it, they can topple over and cause a mini disaster.

  • If you staged your aqua scape somewhere else, it may help to bring in one rock at a time - so you can see exactly where you had it before.

  • Check that they are stable and not moving around by gently pushing them. You don't want to scratch the glass but, if you do, don't worry too much since it will be covered in sand later.

  • Make sure the lid fits without touching the rocks and that they are not blocking the overflow or the return nozzles.

The rocks in my tank ended up looking like this, which is different than what I staged:

I chose to create three separate islands of varying height and leave a sandy, open area in the middle. Only one rock is stacked on top of the others and it feels very stable, so I didn't use any glue or cement.

Things are starting to take shape, now let's get to the sand.