This is a nice plant and somewhat unusual. This pennywort likes to grow out of the tanks. I've seen photos of nice Hydrocotyles that grow underwater, but not this one.
Mine sends out runners along the bottom of the tank, and long stems and a few small roots occur regularly along the runners. The leaves on top of the stems usually grow out of the top of water line. The leaves look like little umbrellas over the water.
This plant blooms regularly. Small white blossoms grow on stems above the water also. There are small blossoms on the plants in the photograph.