There is no single potting recipe for everything sold as pennywort. A pond-margin planting, an aquarium plant and a small pot of uncertain identity are different growing arrangements.
Identify the setup before mixing ingredients
The RHS marsh-pennywort profile places Hydrocotyle vulgaris in moisture-retentive soil or shallow water at a pond margin. NParks describes H. leucocephala as an aquatic herb that can grow immersed or emersed. Neither profile is a universal soil recipe for an unlabelled plant.
Match the medium to the intended use
For a container-grown specimen, establish whether it needs a consistently wet root zone or a draining pot. For pond or aquarium use, follow a method suited to that system rather than tipping an improvised garden-soil mixture into the water.
Record the medium and container you choose. Check how water enters, where it collects and whether you can inspect the plant. More water held around unknown roots is not automatically an improvement.
Review a struggling plant before repotting
Note whether the trouble began after a change in light, container or water level. Keep those observations separate from the decision to change the mix. If the identity remains unclear, use the leaf and stem comparison in Pennywort Types: Leaves and Aquatic Growing Needs before choosing a more specialised growing method.
Photographs for comparison
