Aquarium Gravel For Hydroponics: What You Should Know

With Hydroponics, the soil is subtracted from the equation to be replaced by nutrient-rich water. The roots are generally embedded in a support medium, like Rockwool, expanded clay, or perlite and the nutrients are provided either by a drip or an ebb and flow system.

These support mediums are widely used and have varied price ranges, advantages and disadvantages. Part of their role is to support the plants themselves, aid in nutrient absorption and water retention, and assist in controlling the overall environment so that there are no ph fluctuations outside the accepted ranges in this enclosed ecosphere.

In the end, they can influence factors such as whether there will be a mediocre yield or an abundant harvest.

One media that is often overlooked is gravel. It is not the first choice of many a hydro culturist but it has many advantages, mainly its cost-effectiveness, ready availability, and its durability. Just for those reasons alone many hydroponic growers believe it is worth using as a substrate.

Gravel Advantages and Disadvantages for Hydroponics

The preferred support medium to use in hydroponics is one that does not absorb water, is ph neutral, is reusable and provides good aeration. Gravel ticks all these boxes and then some, coming in a wide variety of shapes and sizes.

Gravel for hydroponics generally falls into two categories, “sharp” gravel or “pea” gravel, and whichever is chosen can sometimes just be a matter of preference. However, whatever type of gravel chosen it is advisable to wash thoroughly to eliminate any debris, dust or dirt particles that can perhaps clog any pumps used.

The distinction between the two is that one is rounded and the other has sharp edges, and a mixture of the two is ideally suited for use as a substrate, generally a mixture of 3/8 to 5/8.

This screening mixture allows particles to filter through and pass their nutrients onto the hanging roots. If the gravel is too densely packed, it would prohibit the free flow of nutrients to the plants and literally starve the crops to death.

What the sharper pieces of gravel tend to do is hold on to nutrients as they flow over the surface areas. This is important because minute particles of nutrients become trapped between the small jagged crevices, and in between the irrigation, the cycle helps to sustain the plants.

Apart from that the rougher surface also provides the roots a better ability to anchor to for stability, where a smoother rock would not, and also the rougher surfaces stops the gravel itself from shifting whenever the water is flowing through and around it.

Occasionally the gravel itself will become soiled and need to be cleaned. Fortunately, unlike some other substrates which have to be thrown out and replaced, gravel can be extracted from the grow bed after harvesting, and any unwanted material that is adhering to surfaces can be washed free. Once free of any dead roots or debris, the now cleaned and sterilized gravel can be put back into position for the next planting session.

This process can be repeated indefinitely, saving a lot of time, money, and effort in transporting the aggregate from the store.

As you can imagine the weight of the gravel can be a disadvantage in transportation and installation and that will reflect in the structural considerations for the hydroponic setup.

Whatever planting structure is used to house the substrate and the hydroponics farm, has to be capable of supporting the weight of the gravel over a long period of time, as well as the weight of the water that also has to be factored into the weight baring equation.

Gravel In Hydroponics And The pH Level                                                                               

The ph level in both hydroponics and aquaponics is constantly requiring recalibration and monitoring to maintain the narrow range. This chemical reaction can occur depending on the type of gravel rock used and that can affect the ph level, raising it higher than the accepted range. If that happens it can interfere with the nutrient absorption as the water flows throughout the system, and adversely affect the plants.

Whether the water is alkaline or acidic can also play a major role in the ph stability ranges and how the gravel will interact within this biosystem. If the water has too much alkalinity, the ph levels can be difficult to stabilize even with the proper equipment and can work out to be a costly endeavor trying to maintain the equilibrium.

On the other hand, too acidic a water source can be aided by the gravel which will actually correct the imbalance.

So, it pays to understand the type of water that is going to be used first in the hydroponic system in regards to its natural ph ratio, as that will help to decide the type of gravel to be selected to avoid any future problems.

Being nonporous, gravel does not have the ability to hold on to water itself. With a drip irrigation system that is not a problem as there is a fairly constant supply of water. However, if using the flood and drain method a back-up system may be required in case of a power cut, or one of the pipes becoming blocked.

And for this reason, with the use of gravel, the watering cycle needs to be more frequent or the roots will dry out and eventually die.

Another item of note to pay attention to is the depth of the gravel which can play a crucial role in the development of the roots. If the gravel is too shallow the roots will not be properly supported and anchored, which in turn will affect the root growing capacity.

If the gravel is too shallow there is also a risk that a layer of algae can form along the surface area which will attract fungi gnats that will revel in feasting on the roots of the crops.

Having a depth of at least 50mm will eliminate this potential build-up as light cannot penetrate to that depth and interact with the moisture to allow the algae to flourish.

The Gravel Truth in Hydroponics

Using gravel as a support medium is ideally suited to an ebb and flow watering system where the aggregate is watered several times a day. It does not absorb nor hold onto the water but its cost-effectiveness and the ability to source it locally offset this disadvantage.

It is important to note that the gravel selected needs to be chemically inert so does not alter the ph level unduly in the ecosystem, so limestone should be avoided. There’s nothing worse than opting for a cheaper support medium only to be committed to doing twice the work in constantly having to recalibrate the ph levels.

Growing crops hydroponically may be a soil-less endeavor but the gravel truth is that it could be the start of a new revolution in sustainable growing, ushering in a new way to feed the world.

And at the end of the day who wouldn’t want to be able to grow more food, faster, and in a completely controlled environment?

What Size Gravel is Best for Aquaponics?

In the same way that hydroponics is a soilless means of growing, so is aquaponics. One of the key differences being the way the grow beds are constructed. While hydroponics systems tend to grow in pots in various systems, an aquaponic system most often uses flood and drain grow beds, which are full of growing media. Gravel being the most common.

What size gravel is best for aquaponics? There are many sizes available, and in theory, any size gravel can be used. Most hydroponic gardeners prefer pea gravel, which is between 1/8 of an inch up to 3/8 of an inch in size. Although, the smaller 1/8 sized pea gravel being the gravel size of choice. 

Considerations of Aquaponic Growing Media

When you first build any aquaponics system, there needs to be a few considerations when choosing the ideal growing media. Here you can find the things you need to think about, and you can see why pea gravel is the ideal media to use.

Inert: Any aquaponic grow media needs to be pH neutral. Similarly, as a hydroponic system, the pH level in an aquaponic system needs to be controlled.

Fish and plants have their preferred range of pH range, and ideally, you need to aim for a range of pH 7. Many media can change the pH level, although pea gravel isn’t one of them.

All you need to do is thoroughly cycle your system before adding fish or plants to clean away any dust.

Grow Bed Depth: Any grow bed in aquaponics should be a minimum of 12-inches deep for most crops.

Weight: Any grow media in use in a hydroponic system should be too heavy to handle. It should be light to medium weight, so you can easily dig your holes when you plant your seedlings. On the flip side, if they are too light, they can float in the grow bed and won’t offer support.

Easy to Work With: Grow media can comprise any size or shape, although sharp edges are best avoided. If you use sharp-edged stones, these can be hard on the hands and damage any grow beds that are not constructed from solid materials.

Non-Decomposing: No aquaponic grow media should break down because it can work its way through your system and clog your water pump and reduce dissolved oxygen in the water for your fish.

Pea Gravel for Aquaponics

Pea gravel fits all the criteria and then some. The smaller pea gravel has a large surface area that bacteria can thrive on. With this, the nitrogen cycle is complete and, in turn, delivers robust plant growth.

From the fish waste, the broken-down compounds create ammonia, which passes around your system. Once it flows through your smaller sized pea gravel, it is converted into nitrites by two bacteria strains (Nitrosomonas bacteria).

From here, these nitrites are consumed by the second strain of bacteria (Nitrobacter bacteria) and converted to nitrates. Your crops then use the nitrates as fertilizer.

As there is more surface area on the gravel, a larger number of nitrates are being produced than if larger gravel was being used.

Pea Gravel with Limestone

Generally, your pea gravel won’t contain anything to change your aquaponic system’s pH level radically. However, if there are any traces of limestone, the results could be different.

Before filling your grow beds, you can carry out a test to check whether your bags of small pea gravel.

Take a handful of gravel, place it in a container, and then cover it with some white vinegar. You can see the vinegar fizzles if there are traces of limestone.

You need to avoid limestone as this will keep your pH above 8, which can harm your plants and your fish.

You can place some gravel in distilled water and check the pH after a couple of days, although it is better not to purchase any gravel you think may be suspect.

One thing to note is that the name pea gravel isn’t a type of gravel, it is because of its size. One dealer can sell something different to another.

Grow Bed Depth and Pea Gravel

One thing any gardener needs to understand when using pea gravel is the depth of the grow bed. The recommended depth is 12 inches, and for a good reason. When building a system, you may be tempted to go for a shallower bed.

Even while pea gravel is the cheapest media you can purchase, the amount you need could be considerable. However, there are reasons why you should opt for a grow bed of this depth, if possible.

Here you can see why avoiding the shallower bed sizes can be beneficial, and it is advisable to scale your system to support the larger bed and the increase in the size of the fish tank you will require.

You will face limitations if you decide to go for a shallow grow bed filled with pea gravel.

Eco-System & Dead Zones

In deep aquaponics grow beds, you naturally create a layered eco-system. One significant benefit of this is that you will prevent any ‘dead zones’ dotted around your grow beds.

Besides this, as you have a thriving environment packed full of beneficial bacteria, worms, and lots of space for roots to grow. You don’t have to clean out your grow bed.

These bacteria and worm communities, along with your plants, do all the cleaning for you. Anything less than 12 inches, and you won’t be able to reap these benefits of a stable eco-system.

Plant Limitations

If you are growing nothing but crops similar to lettuce, you may get away with a grow bed shallower than 12-inches.

However, if you grow indeterminate tomatoes, cucumbers, and corn among the many other crops that can benefit from aquaponics, these shallow beds can’t support them. You won’t receive the same base nutrients and beneficial bacteria for these longer living crops.

One thing not to forget is your aquaponic system can be highly configurable, and you can mix grow beds and use raft systems for short rooting crops and larger grow beds or Dutch becket systems for deep-rooted crops.

Three Zones of Pea Gravel Grow Bed

Zone 1 – Surface Zone

The top 2-inches serve two purposes. It allows for light penetration and acts as the dry zone to limit evaporation. The second reason for this dry zone in your pea gravel is to prevent collar rot and stop any algae forming as the nutrient-rich solution faces light exposure. Also, the infection from powdery mildew is minimized.

Zone 2 – Root Zone

It is in the second or middle layer where the plant activity occurs. Making up this layer is around 6 to 8 inches in depth. As you drain after flooding your grow bed, water drains away completely and presents highly efficient oxygen delivery to the roots, microbes in the soil, worms, and beneficial bacteria.

When you are in the flood cycle, incoming water will distribute all the fish waste particles containing nutrients through the entire grow zone. Worms are highly active in this middle zone, where they break down solid matter to release minerals and add to the underlying worm tea. On each successive flood and drain, this beneficial tea is spread around all your crops.

Zone 3 – Solids & Mineralization

Once you reach the bottom 2-inches, you find all the fish waste solids and worm castings are gathered together. By the time it reaches here, it will have been reduced by around 60%.

In each of your flood and drain cycles, this zone is kept fresh from the effective delivery of oxygen-rich water.


Once you see the benefits of using 1/8-inch pea gravel, you can understand why it is among the most popular media.

It is highly affordable, and any gardener is advised to lean toward the deeper grow beds when possible. Besides this, there is nothing you need to do with pea gravel, and it is one of the easiest materials you can work with.

There is nothing but benefits you can get from using this smaller sized pea gravel than if you used a larger gravel, or an alternative growing media.