Wow these are all great responses, thanks everybody. I think I'm going to replicate what I'm trying to do with a 30 long, I'll build a mini sump and add the dehumidifier water, run co2, and see what it maintains for a ph. I'm not worried about any chemicals or contaminants with the distilled water, just having the water too perfect is a bit of a concern. There will be no dissolved minerals, or any type of buffer.