Aquarium Substrate Layer Cake

If you’re just getting into aquariums, it’s likely that the first and only thought you’ve had about your aquarium’s substrate is: what does it look like? It is equally likely that the second time most people think about their aquarium’s substrate is the time they “clean” their 10 or 20 gallon tank for the first time by emptying everything out of their tank, rinsing out the gravel in a bucket with hot, chlorinated tap water and maybe a little bleach (i.e. more chlorine) only to find all their fish floating dead the next day in their sparkling “clean” tank. The next day they’re talking to the guy or gal at the aquatics shop and find out that those little pebbles at the bottom of their tanks actually served a function by harboring the good bacteria that helped maintain some balance in that tiny ecosystem they were keeping on their bookshelf. The cynical science teacher side of me would also posit that it would also be likely this was the first time that a lot of grown adults heard the words “good” and “bacteria” used in succession in the same sentence, but that’s another conversation for another day. After 2020 just do us all a favor and please, please, PLEASE pay a little more attention in science class, kids.

If the above paragraph describes you, don’t fret. It’s a mistake we all make once, but as long as it’s only once then you’re good to go. If you keep going down the rabbit hole with fish, then it’s likely you’ll learn that the stuff you put in the bottom of your tank actually matters depending on what you’re keeping in the rest of the tank. Cichlids need sand because they like to dig, plants need nutrients to grow, they also need a place to drop their roots, some invertebrates need a place to burrow, and the type of substrate you use could also affect certain water parameters like pH and gH. I can’t say how long I spent reading up on substrates before I set up my most recent tank, but I decided I wanted a little bit of everything, so I did exactly that. I like to call the result my “aquarium substrate layer cake”, which is described below from bottom to top:

Pool Filter Sand - 2 Inches. We need to do water changes not only because our water gets yucky after a while, but also because our fish produce waste in the form of ammonia, and bacteria in both our biological filter media and our substrate converts that ammonia to nitrites and then to nitrates. Ammonia and nitrites are extremely toxic in extremely low levels, but nitrates not so much. Levels as low as 5-10 ppm can adversely affect some fish, but most will be just fine at higher levels and it’s been documented that the hardiest varieties even continue to breed at levels as high as several hundred parts per million.

Our aquariums are closed systems, so we need to remove the nitrates that accumulate by regular partial water changes. Rivers, streams and oceans are open systems, so nature conducts ‘water changes’ via natural currents. Lakes and ponds are actually open systems as well because they are continuous with the groundwater in an area’s water table, so even though there is no visible current, water in these systems is dynamic in the same way that groundwater is dynamic.

Nature also does one thing that we can’t with regular aquarium filters: get rid of nitrates. The reason for this is that denitrifying bacteria live in hypoxic, or oxygen-poor environments. Our aquarium water is an oxygen-rich environment, so the only way to create an oxygen-poor environment is to create a deep substrate that will develop a hypoxic area where these anaerobic bacteria can live. Right now, the tank is home to about 100 very happy guppies, has modest filtration with HOB filters, I perform minimal water changes (25% every 2-4 weeks) and has always read 0-5 ppm for nitrates and 0 for ammonia and nitrites.

The first two inches of pool filter sand serve that purpose. I like using pool filter sand for several reasons: 1) It is cheap enough. Imagine trying to set up a decently-sized tank with a deep substrate and use exclusively plant-specific substrate like Flourite or Eco Complete. You would practically have to take out a loan just to pay for your substrate. Also, since this will be an oxygen-poor environment, plant roots won’t grow in this layer, anyway. There are significantly cheaper alternatives like play sand from a hardware or material supply store, but 2) you don’t have to rinse pool filter sand. If you have the time to thoroughly rinse the cheap stuff and saving the extra money means something to you, go for it, but it’s not worth it to me. If you don’t rinse it thoroughly then your water will be cloudy for weeks after setting up your tank. With pool filter sand you can just dump it in the bottom of your tank and go. Finally, 3) gravel seems like a logical choice for this application as well, but we’re going to be adding a nutritious layer of soil above it, so even small pebbles will allow some of that soil to make its way down to the bottom and away from where we need it. Additionally, larger pebbles would allow oxygenated water from above to get into the hypoxic layer below.

Mesh Screening. The purpose of this layer is to cap off the bottom sand layer and prevent mixing with the above soil layer. It doesn’t have to be perfectly 100% effective, but as long as the mesh is fine enough to prevent most of the particles from mixing, then that will work. I usually find this type of material at a fabric store for about $1.50 per yard and is specifically called “tulle”. You’re looking for something about as fine as a vinyl screen on a screen door, and I imagine you could use the same stuff as long as it’s vinyl and not metal wire.

Topsoil + Osmocote - 1 Inch. Just like your fish eat shrimp and worms and peas (or at least that’s what I feed mine), and you eat cows and chickens and kale and probably some peas yourself, your aquarium plants need organic matter as well to build their structures and tissues. Just like you or your fish can’t eat a rock and expect to get any nutrition from it, the same goes for your plants. You could use some fancy high-priced water supplements, but your plants kind of prefer the good old fashioned way: dropping their roots into some good old fashioned dirt.

Actually, a point I often make with my students: it’s not just dirt, it’s soil. Soil contains broken down organic matter from the decayed bodies of dead plants and animals that future generations of plants take up and incorporate into their tissues. It’s some real circle of life kind of stuff. The soil of choice here is going to be cheap top soil you can get for $1.99 a bag at any hardware store or even grocery stores around Saint Louis in the springtime. Avoid potting soil or anything fancy because it contains extra additives that might be great for growing flowers but that could be toxic to fish. I typically also like to use a dollar stop pasta strainer to sift out all the rocks and sticks and big clods of mud that come mixed in with the topsoil so I’m left over with just the finer loamier stuff. I’ll add about an inch of topsoil and sprinkle a few granules of Osmocote on top of that to give the plants some extra micronutrients. Some people warn against Osmocote because it’s technically not rated for aquariums, but I’ve never had a problem with it.

Mesh Screening. Again, back to the fabric store, only this time in the sewing section. I cap off my soil layer with some pieces of plastic mesh often used in needlecraft. These are sturdier and stiffer than the mesh screening we used below, but are still pliable enough to be workable, and since the holes are larger, they are just small enough to keep the soil layer mostly in place while also large enough to allow plant roots to make their way through the holes and down into the soil. They only run $0.50 to $1.00 for a sheet about the size of a piece of computer paper. I will usually cut these to size, cable tie them together to make one continuous mat, and then lay it on top of my soil layer before putting my top layer of substrate on. The purpose of this layer of screening is to keep the soil from escaping up into your top layer of substrate while at the same time allowing plant roots to grow down into the soil.

50:50 Pea Gravel : Seachem Flourite - 2-4 Inches. Here’s where you are welcome to either follow my formula or exercise as much discretion as you like.  Actually, that applies for the whole setup, but I am just sharing what has worked for me and why so you can replicate my work and enjoy the result without going through the headaches of tons of reading and a long trial-and-error process.  For my top-most layer, I mixed an inert pea gravel with a plant-specific substrate in a 50:50 ratio.  Since I wanted to raise the bed of my aquarium significantly, I wanted a layer that was 4 inches thick in order to get a total depth of 6-7 inches.  If that is not your goal, then feel free to go a little more shallow with this layer.  Two inches should be deep enough for plants to take root, and the screen layer below will provide them a nice sturdy place to anchor their roots.  You could use 100% pea gravel if you like since the nutrients in the soil layer will give your plants a boost, or you could go 100% plant substrate if you like, too.  Since I couldn’t stomach the cost of filling my 55-gallon up with 4 inches of Flourite, I split the difference and bought two bags of Flourite and then ‘diluted’ it slightly with the inert and but significantly cheaper pea gravel.  If the look of your substrate matters to you, then feel free to factor that into the equation as well.  Also feel free to mix and match any of the previous components as you like.  If you don’t have any interest in creating a hypoxic layer with pool filter sand then feel free to leave that out and start with the dirt layer and work your way up.  If you’re not growing plants but want to make a hypoxic layer then do something different on top of the pool filter sand layer.

Conclusion. This is simply what has worked for me with my goals being to make my aquarium both 1) lush and 2) low maintenance. Right now I have a 55-gallon aquarium that is a floor-to-ceiling and wall-to-wall planted jungle full of about 100 very happy guppies, I run no pressurized CO2, have almost no nuisance algae, run only cheap HOB filters, only change 25% of the water every 2-4 weeks, have 0 ppm ammonia and nitrites, have never had a nitrate reading over 10 ppm (they’re usually 0-5 ppm) and for every hour I spend maintaining it I spend countless more hours sitting in front of it and enjoying it, especially since at the time I am writing this it is the middle of January and we are on month 10 of the coronavirus pandemic. As I’ve said in many posts before, I hope you all enjoy your tanks as much as I enjoy mine!