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Getting To The Bottom of Things:
Substrates for Planted Aquaria

By Alesia Benedict

Happy New Year to you all! I hope that one of you New Year's resolutions was either to set up a planted tank or to try your hand with my favorite fish -- Discus! In this issue of Conscientious Aquarist, my column will be devoted entirely to the substrate for planted tanks. In the next issue, my column will talk about various substrate options for keeping Discus fish and we'll explore some of the lesser-known "Do's and Don'ts" of keeping these magnificent underwater beauties.

Planted tanks are unique in many ways, and their needs differ greatly from just "fish only" tanks. One of the major differences is the substrate required. While a few aquatic plants don't even need a substrate, most do of course- and some even need very specialized ones. Substrates serve not only as an anchoring method for plants in which to root, but also as a vehicle for taking in nutrients. For some plants, a substrate is absolutely essential for the plant to reproduce. "Okay," you might be thinking, "so why can't I just pop down to my local aquarium store, purchase some pretty gravel, plant some plants, and bingo, have a happy ending? After all, I'll be purchasing the gravel in a fish store, so it should work fine for my tank, right?"

Well, not really. At least, not exactly. What I mean is, not for a planted tank. The primary problem with that "pretty gravel" that you purchased at the fish store is usually pebble size. This may not matter much to fish, but it certainly does matter to your plants' roots. Pebble matter that is too large will allow water to flow through it too easily -- taking nutrients with it, and allowing debris to collect too easily in the gaps between the pebbles. It also makes it difficult for the plant roots to easily spread through the substrate. On the other hand, substrate with too fine a composition can compact, restricting nutrients from reaching the roots and causing damage to the plants.

The "just right" size is pebble matter of 1-3 mm with a rounded shape. Sharp-edged substrate, no matter how pretty, can damage roots (and fish!) and should not be used. The depth of the substrate is another key factor. I learned this the hard way when first working with some Sword Plants (Echinodorus sp.) I had planted Sword Plants in one of my tanks, and for a while, they were doing great. Yet, just when they got to a certain size, the leaves began to substantially deteriorate and the plant looked like it might be dying. In theory, everything seemed "right" within the tank, but looking at the sword plants, I knew it wasn't. All of the other plants continued to do very well, but the sword plants were now failing. I started to experiment with the lighting, the fertilizing schedule, the water chemistry, etc. all to no avail. In desperation, I decided to move the plants before I lost them. As I gently uprooted the swords, it became obvious to me that the problem was the roots -- they had become tangled, therefore not taking in proper nutrients. They didn't have enough room to expand. In short, the substrate wasn't deep enough! I significantly increased the back third of the tank's substrate and replanted the swords. They prospered! An added bonus was that my Crypts (Cryptocoryne sp.) -- another long-rooted plant -- also took off. In my larger tanks (90 gallons), the depth now slopes from a low of three inches in the front, to just under five inches in the back.

Heating cables like these stay hidden and protected under substrates while they mimic natural temperature gradients between the substrate and water.  Photo by Anthony Calfo.


Trying to simulate Mother Nature within a little glass box is not so easy. In nature, the beds of rivers and streams are usually warmer than the water. Sometimes this is only by a degree or two, but even so, it is enough to create convection currents in the substrate. These currents slowly but consistently circulate warm water up from deep in the substrate (hence, slightly warming the roots) and then back down after being cooled by the body of water. This current means that nutrients are always being delivered to the plants' roots. Heating cables were devised to slightly heat the gravel, keeping roots warm and creating a convection currents. While I don't have any such apparatus in any of my planted tanks, some aquarists find them to be useful when using thin layers of nutrient-laden substrate. This is a great example of how Mother Nature has got this "plant growing thing" down pat, whereas we humans scramble around trying to simulate it via technology.

A note of caution!

Gravel (and tank decorations for that matter) with a high calcium content should be used, which means leave the limestone or coral based products on the shelf. This includes -ack!- all of the beautiful sea shells you've collected over the years while vacationing at the shore!!! Such items will increase the pH and alkalinity, and your plants will surely suffer. So what can you use? Actually, as you'll see, you have numerous choices, and there are pros/cons to all.

Quartz Gravel

This gravel comes in a bunch of colors and shades and is perfectly safe for all aquatic life. It is limestone-free, contains pebble sizes of 1-3 mm, and if you push your fingers into it, it yields effortlessly which means that roots can easily develop. If you have a tank in which you are going to have deep-rooted plants, quartz gravel alone will not typically be enough, as it is a completely inert substrate. However, I have used it as the sole substrate material in two of my planted tanks which house Vallisneria and shallow-stem plants. The plants are "fed" through water changes (mineral traces), fish waste, and left over fish food (both which contribute organic matter). This is enough to feed these plants' roots, and I add nothing to the tank.


Sand has been used extensively in planted tanks over the years -- and with good results. If you want to use sand, make sure it is safe for the aquarium. Some commercial sand has traces of lime, and we've already learned plants don't do well with lime-based substances in their tank. The most common and well known sand used in planted tanks is "silver sand". Many aquarium shop owners swear by it. However, really fine-grade sand can compact over time and become anoxic. This can be lethal to aquarium inhabitants, so I suggest that fine sands be added over a layer of gravel.

Aquarium plants rooted in an artificial nutrient rich substrate.  Note the reddish color from high iron content.  Photo by Anthony Calfo.

Nutrient-rich Choices

These substrates and additives can be a huge blessing if you are cultivating deep-rooted plants or those that need an iron-rich substrate. Heavily planted tanks, and/or a tank composed of "high maintenance" plants may also benefit. The two additives that are most commonly used are peat and Laterite. Peat is rich in organic matter and provides plants with a high dose of nutrients, which can work wonderfully under ideal conditions. I tend not to recommend peat as a substrate additive, because it is tricky and under less than ideal conditions, algae can become more of a nuisance than ever imagined. Laterite is a time-honored additive, and it is mostly found in fine powder form. One can either mix it to the very bottom section of the gravel, or layer it very thinly, with a sizably deep portion of inert gravel on top, which prevents the Laterite from leaching out into the water. Another way to get Laterite and other nutrients into the substrate is through the use of fertilizing tablets. These tablets are pushed directly into the gravel, typically on a monthly basis.

Over the last several years, plant specific substrates have come onto the market. Flourite, Floramax, Eco-Complete and Flora Base are some examples of these. My experience has mostly been with Flourite, and I have been quite happy with the results, though I have also used Eco-Complete with good results. I do not have first-hand experience with the other aforementioned substrates, but would welcome feedback from readers who have. These substrates work by releasing nutrients over a very long period of time, continually feeding plant roots. When I use Flourite, 90% of the substrate is Flourite, and the remaining 10% is simply a neutral-colored quartz gravel layered on top. This top layer helps to show off the fish as well as keep the "dust" down when planting and re-planting.


Sterilized potting soil (not regular dirt from your garden!) can be and excellent choice for part of a planted tank substrate, but it is very, very tricky. Care must be taken to choose potting soils that are void of fertilizers and/or additives. Potting soil is very rich in organic material and the tank will often go through many changes in water chemistry while the tank settles. About an inch deep layer of potting soil should be laid and about an inch of quality gravel laid over it. One important benefit is that there is an ongoing breakdown of organic matter which results in the continually low level release of CO2. Hence, CO2 injection, substrate additives, fertilizers, etc. become unnecessary in many of these aquariums.

There are several downsides to potting soil substrates. The aquarium can experience nutrient release spikes which can be very dangerous to fish. This is especially true for the first few weeks following set up and for this reason, fish should not be added for quite some time. Filtering with carbon is also often required. Also, for aquarists who like to move their plants around a lot (as I do), this is NOT a good option for you as the soil will be kicked up into the water column routinely.

Now that the substrate is laid, the tank is planted, the fish are swimming and all is well. What should be done with the substrate in terms of maintenance? Leave it alone? Vacuum it? Vacuum only the top? Well, it depends. UGH! I know I hate it when someone says that (tell me precisely the right method!), but it really does depend the tank. For the most part, the substrate should pretty much be left alone -- resist the urge to do dramatic things to it if all is well. In time, organic matter will collect in the substrate and create anaerobic conditions and make the substrate more dense. In a non-planted tank, this is reason to start freaking out, and a good gravel siphoning is in order. Yet in a thriving planted tank, the organic matter is going to be broken down and taken in by the plant roots, which will in turn release small doses of oxygen into the substrate and prevent it from stagnating.

If, like me, your planted tanks have some areas of gravel showing, then I recommend siphoning/stirring the top layer, because mulm can accumulate. If, as in most cases, there are no plant roots at the surface, it is best to remove some of the debris from this uppermost layer. Do not, however, dig the siphon into the gravel to suck the substrate clean! In my early planted tank days, I did this in a spot or two and I was horrified at what the siphon was removing! My mistake was that I was applying the principles of a fish-only tank to a planted aquarium. I have since discovered that it is only when that "dirty layer" starts to become quite visible that the plants really start to take off (it appears about an inch or two below the substrate surface, and you'll actually see the layer). I do still siphon, but more to remove mulm, dead leaves, etc., than to try to suck the life (literally) out of the substrate!

Until next time, keep it underwater!

WWM on Substrates for Planted Tanks

Related Articles: Substrates for Freshwater Planted Aquariums, Soil Supplements for Freshwater Planted Aquariums

Related FAQs: Substrates for the Aquarium GardenUse of Soil in Aquarium Gardening,


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