Yeah DG!
You've got it, it's one of a pair of cold water weather loaches. They have acclimatised themselves to trop. waters now and to put them back in the pond would surely kill them. They seem very happy with the rest of their tankmates.
Sorry if this is a stupid question, but are those live plants? I am trying to figure out what to do in my tank as far as plants... thanks, and 3rd TOTM nomination here...