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Mixed Reef Aquarium Aquascaping

Custom Reef Aquascape

When I design a reef aquarium, I think about three things: (1) What is in the best interest of the animals I’m keeping, (2) what looks the best, and (3) what is most convenient and easiest for me to work with and maintain.

Whether it’s equipment, aquascaping, filtration, lighting, or the tank and stand itself, these three factors, in that order, are what I consider a priority. Sometimes they don’t mesh well. What is best for the animal may seem inefficient or difficult for me. For example, I may not find it convenient or cost effective to replace filter socks every couple days, but if that is what the animals require for their health, then I must oblige.

More often than not, there is a method to maximizing animal health and happiness that also agrees with my convenience. For example, roller mats are available for those with high physical filtration needs, but can’t replace filter socks consistently.

Within the scope of aquascaping, I favor a method that maximizes fish and coral health and happiness, and beauty and naturalism, while also efficiently providing convenience and ease of accessibility for me.

The aquascaping design I discuss below utilizes Marco Rock to create elaborate, customizable structures which are built separately for ease of movement, relocation, and installation, but when placed together in the aquarium, look to be one entire structure. You can use many different types of rock, but for this article I’ll mainly discuss Marco Rock.

Why Build Your Own Custom Aquascape?

This design is referred to as Habitat Negative Space Aquascape (HNSA) and provides the following features:.

  • Diverse offering of fish and invertebrate friendly habitats (caves, overhangs, burrows)

  • Plenty of varying spaces for coral placement

  • Customizable

  • Strong and durable

  • Realistic and natural

  • Easily separated

These features provide the following benefits

  • Stress free, healthier, and happier fish and inverts

  • Corals can be placed exactly where they will benefit from proper light, flow, and distance from neighbors

  • Can build the scape to fit your tank, lighting, and flow perfectly

  • Will not break or collapse when installing, moving, or rearranging

  • Provides a natural and realistic aesthetic to any aquarium

  • Easy to remove when catching difficult fish is necessary

While most commonly used for reef tanks, this style can be adapted to nearly any aquarium. You just have to use the right rock and design the most natural and realistic aesthetic. A river aquascape won’t use the same design as a reef crest.

How to build a Habitat Negative Space Aquascape

The process of building a Habitat Negative Space Aquascape (HNSA) begins with knowing the dimensions of your tank. Begin by measuring the height, depth, and width of the tank. Next, you’ll need to construct a frame with these dimensions within with you will build the structure.

This step is the most crucial. Building your structure without a frame that matches your tank’s dimensions will result in building an aquascape that is either too large or too small.

The best frame is constructed by cutting and joining pvc tubing to make a 3d “frame” of the tank, then build your structure within the frame. An alternative is to cut a piece of cardboard with the same width and depth as the tank. Then apply a piece of tape at the correct height to the wall behind your workspace.

Reef Hardscape

Plan Ahead

Once you have your frame or outline built, consider where your overflow boxes, returns, powerheads, and other equipment will be located. This build is completely customizable. You can build a structure that will allow for perfect flow by designing around your powerheads and returns.

Also, keep in mind, with reef tanks, the rocks should not be taller than 50% the height of your tank for SPS corals, and no taller than 70% the height of your tank for LPS corals. This will allow room for them to grow.

You can also build one section at 50%, another at 60% and a third at 70% total height, for example, if you plan on a mixed reef.

Materials for a Mixed Reef Aquascape

The next step is to gather your materials. For this article, I’ll be using Marco Rock as that is what I prefer. It is lightweight, clean, porous, and easy to chisel and shape. I find each piece, or section, of the overall structure to weigh approximately 15 to 20 pounds for medium tanks (50-120 gallons) and 20 to 40 pounds for larger tanks (120 + gallons).

Cuts of Marco Rock

There are two or three types of cuts of Marco Rock you’ll need for this scape. If you want your build to have a shelf or two, then you’ll need a piece of shelf rock. This is a shape that is nearly flat on both sides and works as a jutting shelf. Not all builds need this shape, I’ve only used it a couple times.

The second type of cut is a foundation cut. This is a regular Marco boulder that has been machine cut perfectly in half so one side is perfectly flat and the other is the normal, irregular shape. These cuts are used as the foundation as they lay flat on the ground and they are very stable.

saltwater reef aquascape

You can purchase foundation cuts precut, or you can purchase boulders and cut them yourself using a diamond blade and table saw. I have not found a large enough saw to cleanly cut a medium sized Marco Rock in half without having to rotate the rock. However, it is cheaper to cut them yourself if you have a larger project.

The third type of cut is the regular Marco boulder. These are not cut or shaped and come in many shapes and sizes. They are usually sold in a few different sizes or by weight. Unless you are designing a massive project, the medium sized 8-14” rocks work well. They’ll be broken up into smaller pieces later, so get larger pieces than you think you’ll need.

In addition to the rock, you’ll need the following materials

  • Extra thick super glue

  • Insta-Set spray

  • Two part epoxy adhesive (Seachem coral crete)

  • General bonding glue (watery, liquid super glue)

  • ⅝ chisel and hammer

  • Wire mesh sieve or colander

  • 1 ml syringe

Per 20 pounds of structure, I recommend 2 to 3 ounces of extra thick super glue, 1 ounce of Insta-Set, 1 ounce of general bonding glue, and 16 ounces (4, 4 ounce sticks) of epoxy.

Constructing the Hardscape

After you have laid out the frame for your tank and gathered your materials, the next step is to start building the structure.

Foundation Layer

Begin by laying out your foundation rocks. These are the cuts with the flat bottom and normal, irregular top. Arrange these rocks in different combinations. Be sure to turn and rotate each one, move them back and forth, and change them up all together. Do this several times and you'll start to see which ones look good together and which pieces are the most solid.

reef hardscape build

The general shape you're looking for can vary depending on the size of your tank, but generally, you want a wide, arching foundation so you can build overhangs from a single point and the foundation will support the weight.

When you think you’ve got the right arrangement, leave it out and come back to it a couple times before gluing. You may find a few changes you want to make, and now is the time to make them.

Gluing the Rocks

When you’re ready to glue the foundation together, begin by applying the extra thick super glue along the top joints, where each rock joins together. Smooth out the glue with a wooden skewer or similar tool. Smoothing out the glue covers more of the rock surface around the joint. Then, spray the Insta-Set over the glue; just enough to wet the glue. It should crust over and hold within seconds.

As you progress along the structure, applying glue, smoothing it, and spraying Insta-Set, do not move the piece. Even though the Insta-Set hardens the glue immediately, it is still soft in the center. The foundation rocks moving away from each other, even slightly, may compromise the structural integrity of the whole structure.

Before you glue a rock to the rest of the already glued rocks, you can adjust it slightly to make sure it is a tight fit against the neighboring rock before gluing.

Applying Epoxy

After the top of each joint has been glued, let it sit for 24 hours. Next, apply the epoxy to each joint. After activating the epoxy by rubbing and folding the two parts together, shape it into a long strand, the length of the joint. Apply a strand to each joint and press the epoxy into the rock. The epoxy needs to make contact with as much rock as possible; use as much as you need for each joint. The epoxy should fill in as much space in the joint between rocks as possible.

The epoxy sets within 5 minutes after mixing. Be sure to work fairly quickly when applying it. After you have applied and pressed the epoxy into the rock joints, stamp a pattern into the epoxy using a small piece of Marco Rock. This will blend the epoxy texture with the rock and it won’t look like smooth epoxy, but part of the rock itself. Later, we’ll deal with the difference in color as well.

Let the epoxy cure for 24 hours and then carefully flip the entire structure over and repeat the gluing and epoxy process for the bottom of each joint.

At this point, you should have glue and epoxy covering the top and bottom of each joint where the rocks connect. This portion of the build takes the longest as you must wait approximately 24 hours for the application of each stage of glue and epoxy to cure, approximately 96 hours total.

Chiseling and Shaping Rocks

Now that we have a solid foundation on which to build, it is time to shape the rocks that will be used to build the rest of the structure. Gather your regular Marco boulders, a ⅝ chisel, hammer, safety glasses, and a mat, tarp, or piece of cardboard on which to work.

Examine the first piece of Marco Rock and look for any areas where a break would result in an interesting shape. If the rock is shaped fairly uniformly, then apply the chisel in the middle of the rock and simply split it in half.

Continue to chisel the larger rocks into interesting and irregularly shaped smaller ones. The more irregular, twisted, and arching the better.

Depending on the size of your tank, you’ll need pieces of rock that range in size from small to large. You can always glue two smaller rocks together to form an interesting shaped larger piece if you need.

As you chisel away, you’ll get lots of very small pieces you did not intend to break off. Keep these as they will come in handy later. It is also crucial to keep any and all dust and particles that are collected by the mat or tarp. Sweep up the dust and smaller particles and save it for later.

The Second Layer

In my experience, the best way to proceed is to lay out as many of the newly chiseled rocks as you have room for and begin to orient different pieces on the foundation. The second layer should be open, with lots of larger spaces for fish to swim through and span the length of the foundation, allowing lots of space for the next layers to be attached.

Habitat negative space aquascape

To accomplish this, I like to use more elongated pieces and orient them upright to create several columns. One of the benefits of this build is the structural integrity. This allows you to build far reaching overhangs which are unsupported under one side, but the foundation allows for the proper weight distribution to support them. Without proper weight distribution, you would have to forgo overhangs and use arches only.

designing a reef aquascape

Overhangs are more natural, realistic, and downright cool in my opinion. They also provide the surface area on which to put corals and the cover for fish habitat without requiring two support rocks underneath. Having said this, don’t avoid arches all together, a few well placed arches can look good, I would just favor overhangs overall.

When trying different rocks and orientations for your second layer, leave at least one of the terminal foundation rocks, if not both, with nothing built upon it. This piece or pieces will serve as a weight anchor, where your largest overhangs will be built above. If you put a secondary layer column on the terminal rocks, the whole build will appear too dense and everything will be an arch instead of overhangs.

designing a reef hardscape

It is difficult to envision the entire build at this stage. The second layer is more about creating options moving forward than specific design features. By all means, choose cool rocks with interesting shapes to apply to the second layer. However, the priority should be to keep the second layer fairly open for larger fish to swim through, and to create several surfaces for the third layer to build upon.

At this stage, whether or not there is a surface there at all is more important than the shape, size, or orientation.

When you decide on the rocks and their placement for the second layer, apply glue to the joints just like with the foundation, wait 24 hours, and apply epoxy.

Designing the Third Layer

In terms of design, the third layer is the most important. The design of this layer determines how the overall structure is shaped. It is also the layer on which the rest of the structure is built upon.

customizable saltwater reef hardscape

When I say layer, don’t picture horizontal layers like a cake. The term stages could be used synonymously with layers in this case. The third layer is such because it is built after the first two layers, but spatially, it can cover the entire structure. Some rocks on the third layer will be at the very top, others will swoop down near the foundation.

Designing this layer is all about thinking ahead. You want to attach rocks in locations and orientations that will facilitate the overall shape you're aiming for.

Building the Third Layer

Gluing during this stage is a little trickier than the others. Some of the rocks will be overhanging and have to be held with one hand and glued with the other. To accomplish this, have your glue and Insta-Set uncapped and ready. Orient the rock where you want it, then apply a layer of glue to the top of the joint, smooth it out, and spray the Insta-Set. Hold for 10 seconds and let go. Apply a little more glue if necessary.

customizable saltwater reef aquascape

Wait to epoxy any joints until after you’re finished building this layer; you may need to remove rocks. Also, epoxy will only be applied to the bottom of joints from this point on.

Only glue a few rocks at a time. Take a few moments to step back and look at the structure from a distance. After you’ve done quite a bit of building, come back to it the next day, with fresh eyes.

I also found setting the structure upon a turntable of some kind helps with seeing the structure from multiple angles. Keep the structure within the framed boundaries of the tank. When you're finished building and it's time to apply epoxy everywhere, the turntable is incredibly useful.

building a saltwater reef aquascape build

Applying Epoxy

After you are satisfied with the shape and build of the structure, it’s time to apply epoxy. The epoxy is responsible for a majority of the structural strength of the build. There is already epoxy applied to the foundation and second layers.

The underside of each joint in the entire third layer requires the application of some epoxy. The amount depends on the open space in between the joined rocks. The super glue acts as a binding agent, the purpose of epoxy is to fill in the gaps and provide support where there is none. Anywhere there is empty space underneath, where two rocks are joined, apply some epoxy.

Just like with before, apply the epoxy within a couple minutes after mixing, press into the gaps, and stamp it with a rock to create texture.

Finishing Touches

The final step is to cover the epoxy with Marco Rock dust. This will hide the epoxy’s color and result in a uniform and natural look.

habitat negative space aquascape

Begin by collecting all the dust and small particles you collected from chiseling, breaking up rocks, etc. Place a colander over a bucket and sift until nothing else falls through. Place a mesh sieve or colander, with smaller holes than the first, over a second bucket and pour the first bucket into it. Sift until only large particles remain in the sieve.

If your mesh sieve is small enough, you’ll have only dust in the second bucket, if your dust is mixed with particles larger than a pinhead, sift it again through a smaller gauge sieve. You want a powdery, sandy, dusty mixture, not gravel.

Next, take a piece of paper and shape a narrow cone tube with a tapered end. This will be your applicator. Dust goes in one end and can be applied with the narrow, tapered end.

Then, use the 1 mL syringe and apply the general bonding glue (watery, liquid super glue) to a patch of epoxy until it is covered. Then apply some dust to the glue and press in with a nitrile gloved finger. Repeat these steps once or twice more until the epoxy is covered in dust. Move on to the next patch and repeat until all epoxy is covered. You can apply glue to multiple patches at a time if you work quickly enough, but I prefer to take it one at a time because I apply multiple “coats”.

Once your glue and dust has dried, spray the structure with some pressurized air to remove excess dust.

Installing in the Aquarium

Now your structures are ready to add to your aquarium. If you're installing it to an existing aquarium, remove several gallons of water before adding the structure. If you're replacing existing rock with this structure in an established reef aquarium, only remove a portion of the old rock, not all of it. You’ll want bacteria, microfauna, and macrofauna to transfer to the new rock before the old rock is removed.

These structures should be very stable. Orient them in your aquarium how you see fit. If you find some places lacking, add mini structures to supplement the larger one. If you find it is too dense, with sufficient pressure, you can remove individual pieces of rock until the shape is more to your liking.


Don’t be afraid to experiment with the design. You can always break rock off and go back a few steps as you build. Take your time, stand back, and examine your build with fresh eyes periodically.

The point of this style of aquascaping is to create sufficient habitat for all types and sizes of fish and invertebrates, while allowing ample surface area for corals, keep this in mind as you build and you can’t go wrong.

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