Carpeting plants stay low and spread across the substrate to make a lawn. Dwarf Sagittaria and Dwarf Hairgrass are the easier ones, while Monte Carlo, Dwarf Baby Tears and Pogostemon helferi usually want strong light and CO2 to fill in tight. Plant in small, spaced portions and give them time to run. Staurogyne and Hydrocotyle are good in-between options if you want a lower look without a full carpet. Trim often so the lower leaves still get light. Check each listing for what's in the bag, and note that anything labeled emersed may drop some leaves while it adjusts to growing underwater.