Mike -
I have noticed a slight increase in evaporation in my Nano that has two Mangrove pods, as well. I have crushed coral as my substrate, and only partially barried the roots in the substrate. The mangroves seem to be doing fine... and my water quality parameters have drastically improved since adding them. :-D I'm adding about a cup of (extra) ro/di to the tank, every couple of days to keep the salinity at a reasonable level.

HTH