That does look like a deformity. I think you are starting to get into quality of life issues here, especially if the front legs are also deformed. Are these tads that you pulled from a local stream/pond? If so, you might be right about heat being a problem. Ordinarily in situations like this, we would probably ask you a bunch of questions about the husbandry of the parents, but that is a non-issue if you pulled these out of a local stream or pond. Temp is something to look at and maybe what you are feeding the little guys, but that is much less likely. If this is happening to more than one tad, though, it is likely environmental somehow.
is an article by Ed that I think could be relevant for you. Sorry you are having to deal with this.