if i had to take a stab at it i'd say acmella repens though I'm probably wrong. I'd also get rid of the pink gravel, does you absolutely no favors. Look up your local landscaping company and buy a bag of turface. Even play sand would be better.

 

http://www.aquaticplantcentral.com/...ral.com/forumapc/plantfinder/details.php?id=311&category=genus&spec=Clinopodium

Stem cross section will be square and the plant will have a minty smell.

Ditto on the pink gravel...

 

Thanks for helping and I just converted that tank from that hideous red and black gravel to Leslie's Pool Filtration Sand today. Took me awhile to do it.