They are the same species as a Neon Tetra, so mix just like you would a Neon.
, but my diamond (head?) are definatetly tetra. I have eight; they are very like the cardinal and the neon, but they have the neon green eyes and no neon stripe. My green neon little, little guys usually school with them. I always asume that the tiny green neons get some extra protection. They are a community fish. Too small to harm any one. I've seen very little fuss, an occasional quick dart at a fellow to tell him off. (In a serpae it would be a fin nip.) They school like a cloud drifting. They are in my very varied tetra 100 usg with cories, otos, clown loaches, dwarf cichlids. They are my base community stock. Not the showiest, but lovely. I gotta tell ya, short of correction by an academic or a breeder, I'm sure they are tetra. And easy to keep, as long as you don't house them with an oscar. 
) Gottcha, The Wolf. My Bad! (as the kids say.) I started wondering if I might have lost your thread of reference. So if the original poster is still around maybe someone has a good site or can provide some links to compare so IMCL85 can find out what's swimming in the tank and some care instructions. I haven't seen any pictures of the diamond head, but a red/grey neon is close enough, I think. The picture on the web site that Elisabeth85 posted in the general Chit-Chat Forum has a good one.
