Everyone loves carpets. However, as smaller plants that grow on the substrate they have less access to light and are more delicate than larger, robust plants. With a little specialized attention, various carpets can be grown with high success rates in a low tech tank.
Growing a carpet without the help of CO2 injection is like trying to build muscle with limited access to good protein, or swimming without wearing goggles. It is definitely possible, but success becomes highly dependent on overcoming 3 challenges:
Carpets need decent lighting to reach them as they lay at the point furthest away from the tank, on the substrate but many low tech tanks choose to use low lighting due to fear of algae. Many of the more attractive species of carpeting plants grow better with more light.
In nature, many aquatic plant species grow in CO2 rich environments, but here we want a "low tech" setup that does not use CO2 injection to replicate such a condition.
The most difficult stage of growing a carpet is at the start. There are various factors that contribute to transition stress for newly planted plants; if you bought them in emersed form they have to transition to submerged growth forms, if they came from different water parameters they have to reprogramme their enzymes to fit the current tank environment. The chances of success rates are maximized when transition stress is reduced.
By following the guide below, you should be able to grow dwarf hair grass carpet (DHG), Monte carlo (MC), Microsword, and Marsilea species without injected CO2. Glossostigma may require more light. HC Cuba plant is tough to grow without CO2, and should not be attempted unless you have spare time and plants to waste, but this approach will increase your chances massively.
Examples of low tech planted aquariums with green carpets:
The dwarf hair grass carpet above in my 3 gallon tank is grown completely from scratch and took around 3 months to fill in.
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A shallower tank ( I recommend tanks 12 inches or less) allows for stronger lighting at the substrate level without an expensive lighting system and allows for a less steep light gradient.
Shallower tanks also tend to higher soil to water volume and better gaseous exchange. They are easier to manage as a whole; easier to access which encourages maintenance.
The carpet of glossostigma in the picture below is grown using 150w Metal Halides hung 2 feet above the tank. This light level is probably excessive; but it does demonstrate that high light + no CO2 doesn't always translate into an automatic algae-fest - that is, if the plants are growing well. The guide in the algae section of this site outlines how to maintain an algae-free tank without the use of heavy chemicals.
In your light choice, aim for a PAR value of above 50 Umols at substrate level. I use close to 100 myself.
To find out how to read a PAR table, go here. As an example, if you are using a tank that is 12 inches deep with 2 inches of substrate, the relevant depth is 12-2 = 10 inches. This will put you in what many source will call "high light" territory. It does come with increased risk of algae if you do not control tank parameters well. I recommend reading through the guide to avoid algae - something you will need to master if using high light levels in a low tech tank.
Using stronger light highly increases the chances of success with carpeting plants in low tech tanks.
Here I use a 23watt CFL desklamp to grow dwarf hairgrass in my low tech tank. The adjustability of the desk lamp's height allows me to cast more or less light into the tank.
There is an abundance of substrate choices, as detailed here. To grow a carpet in a low tech environment, go for topsoil or plain garden/potting soil without additives. A balanced loam soil - roughly equal amounts of clay, silt, sand and organic matter with some peat content will do as well. Organic content of the soil should not be higher than 20%. Soils that are too labile create more stability issues due to over-active decomposition. If you use very organic-rich soil, use about an inch or less. Commercial aquasoils are a good option that removes guesswork, and are much easier and less messy to manage, but they cost a lot more.
Soil decomposition provides low levels of carbon. It will not make up for the lack of CO2 injection entirely, but provides enough to grow easy carpets. Carpet plants also root better in soil.
Just for growing carpet plants, just a thin layer of soil is sufficient. A 1 or 2 inch layer of soil, followed by a thin cap of 1/3 or 1/2 of an inch works well. You can use a thicker cap if you are afraid of stirring up the soil base during replanting.
In this example, a thin sand cap is used over a raw soil base.
In this low tech example above, I grew HC using an aquasoil cap over raw garden soil. Aquasoilactually makes a great cap; it stays on top of raw soil easily, whereas heavier sand sometimes sink if the substrate is disturbed.
Many plants fail to grow, not because growth conditions are entirely lacking, but because they cannot withstand the transitioning process when transferred from another tank with very different water parameters.
In CO2 injected planted aquariums, the quick boost in growth allows plants to adjust rapidly. In non-CO2 injected planted tanks, plants have to do the tough adjustment without the help of sufficient CO2. There are 2 key parts to reducing transition shock:
Step 1: perform full pre-cycling
This means completing ammonia cycling for the tank, and allowing the tank bacteria colonies to mature, typically letting the tank's filter run and the water to settle over 2-3 weeks. This process can be hastened if using starter bacteria culture to about 1 week. This step is often ignored by eager aquarists who are impatient, to great peril. Ammonia burn young and sensitive plants easily.
Volatile organics also trigger algae. Giving time for the tank to become biologically mature reduces algae issues. This is an important point especially if one is starting out using higher amounts of light.
For detailed steps on how to cycle a tank, click here.
Step 2: go local
Getting plants from the same water parameters as the tank environment greatly reduces transition stress & melting. Alkalinity (KH values) have a larger impact than other variables; staying within a 3 dKH range is very safe.
If you want to avoid the hassle of actually measuring water parameters, get plants that are from your state or province: there is a high chance that the water parameters are quite similar to the water from your tap. Good vendors should know where their plants are grown / where they come from. Buying plants that are shipped from far away places bear significant risk that the water parameters may be very different, increasing the likelihood of transition shock / death.
Carpets should be spread across the substrate thinly rather than planted in thick clumps. ( commercial youtube channels give a lot of negative examples here by planting in huge chunks ).
Planting in smaller clumps allow each individual clump to better receive light, nutrients and flow and plants root better compared to starting out in a crowded location. Individual plantlets also root better.
This is HC Cuba plant in a 5 gallon high tech tank. Planting technique is similar regardless of CO2 usage.
For list of best carpeting plants for planted tank, click here.