I don't know but you should be okay with alot of them...Egeria densa and A. Crispus have dCH thresholds of 18 while others like Bacopa Car. and Cryp. Wendtii will thrive in 15.
But all plants in the same group will not react uniformly... A.rigidifolius is a nono with 3-5 dCH and trumpet Crypt at 8.
Why don't you add some peat granules to your filter? That will soften and add healthy tannins to the column (althouh some people don't like the stain).
p.s. For plants, total hardness is less important than carbonate hardness.