Tom Knight meant to write:
The molecular size and charge distribution is almost identical, so the crystals form without distinguishing between H2O and DHO. It would be rare to find any significant number of molecules of D2O.
[I corrected a DH2O to DHO, per later conversation.] Let's be clear here: heavy water is indeed D2O. You can't have a sample of pure, um, welterweight water, DHO: the hydrogen atoms are exchanged between water molecules all the time, so you'd really have a sample with half DHO and a quarter each H2O and D2O. Per Steve Gray's density calculation, you would need about (1/.914 - 1)/(20/18 - 1) = "85% heavy" water to make ice the density of normal liquid water. -- It is very dark and after 2000. If you continue you are likely to be eaten by a bleen.