3 challenges to overcome
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:
LIGHT
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.
CO2
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.
ACCLIMATION
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.
The Possibilities
By following the guide below, you should be able to grow dwarf hair grass carpet (DHG), Monte carlo (MC), Microsword (Lileopsis species), 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.