Thank You, jaynine............It does look similar to the Pycanthemum tenuifolium, but with much softer, whiter, and fleshier stem. Thuis is definitely a water plant......As I remember, (My mom used to have the Mountaiun Mint), it's a stiff, upright plant that grew on dry land. This would not survive on dry land, and the stem is trailing......horizontal.

Or better still Spermacoce glabra? Here is a link that might be useful: Spermacoce

Closer view might improve results for you.

You really can't put too much trust in Internet photos for species that are sold for aquariums, because they are often entirely different from the same species in the wild. Leaves for Limnobium spongia can be floating, or emergent, and plants could easily be found on mud if water depth varies. Since I've never seen L. spongia, I'm not stubborn about the ID :). However, you might find the following link useful, since it provides a detailed description of that plant - Limnobium spongia from FNA.