This is a wonderful little fern. It's one of my favorites and makes a great ground cover. In most of my tanks it grows with the single leaf form across the bottom of the tank. Sometimes in the back of the tanks under the other foliage some of the 4 leaf forms will come out.
Lighting might play a part in which leaf form grows. Usually under lower light the 4 leaf forms grow, but I do have one tank that seems like it doesn't get much light that is covered in the 2 leaf form. Below is a photo of both types of leaves in one tank.
There may be some question in regards to the species. Several similar, looking but different species of Marsilea are probably being offered to aquarists.