a bit late, but basically there are two species of discus.
the commonly known Heckel discus stay slightly smaller than the 'green' discus ( many common names given for both)
but even the heckel will grow to about 6".
Not for the begginer fish keeper.
Dan
That is true, but what you failed to mentino though is that Symphysodon are harder to keep than Symphysodon aequifaciatus.