"his tail is redl"? Do you mean your fish, or the one in the picture?
The fish in that picture is a Pristella, which is also sometimes called an X-Ray Tetra. I've got three of them, by mistake. They were in the same tank as some lemon tetras, and I wasn't paying attention as the shop guy was netting. Of the three, the redness of the tail varies a bit. One is not very red at all, at the moment.
Try this picture:
http
/www.aquahobby.com/gallery/gpristella.html
Mine seem to do just great in my hard water. Currently, my 3 X-Rays and 3 Lemon tetras are assigned to "keep the Q tank cycled" duty. They seem to shoal pretty well with the lemons, but every once in a while (say, a couple times a month), they'll seem to break up into "species gangs" and harass each other (in a playground fight sort of way) for about half an hour, then it's business as usual.
On a side note, I dread the idea of having to net them again. They are by far the toughest fish to net I've ever seen.