Eelgrass clumps (Zostera marina)

Tidepool Tim says, "Many marine invertebrates, like snails, love to hide in eelgrass. Seagrass is also great in tanks because it provides the environment with oxygen. Water that's rich in dissolved oxygen is more apt to support both animal and plant life.  This grass will even add oxygen to the aquarium substrate with its root mass. Some tank dwellers, like urchins, will feed on your growing eelgrass."