Did a google search - this is what I found on a fw forum:
"Many, if not most, dehumidifiers use copper coils for the condenser.
The low-salts (essentially distilled) water that condenses on them is
very corrosive, and may well pick up some of that copper. Topping it
off, the unit passes a lot of room air over that water, so room
deodorants, cooking fumes and oils, insecticides, paint, smog and all
other household air pollutants can add to the "stuff" in solution. I
wouldn't drink it, and I certainly would only use it for fish in a dire
emergency. [That is, it's better than being dry!]"
credit: Wright Huntley 1997
While technology has certainly advanced the past 10 years, I would say the comment about the room air infiltrating that water likely still applies.
Another guy on that same thread mentioned breeding guppies in dehumidifier water and the brood came out half dead with the other half mutated. Not good. This even though the adult guppy appeared fine.