I researched this subject quite a bit for another member a while back. There are some minor and major drawbacks to using reclaimed water from a dehumidifier. If the unit is new, the water is relatively safe. But, even new units have to be checked for suitability of using the condensed water due to anti-oxidant coatings from particular manufacturing plants. IMO, it's just safer to use the water on outdoor plants then to use it in aquaria.
Dehumidifiers:
-with copper coils can have high enough levels of copper in the condensate to kill off all inverts and harm the livers of many fish and herps.
-Any oils/chemicals in the air will also be concentrated in the condensate tank.
-Unless cleaned every couple of days, the mold commonly found in the cases and condensate tanks of dehumidifiers will be more concentrated than the aquarium is normally exposed to from the ambient air.
A note on dehumidfier copper coils versus home plumbing copper pipes:
We've had members experience problems upon moving into new construction homes. Same water, same plumbing, and same municipal reservoir. However, their fish/inverts suffered inexplicably. I asked a civil engineer for the county water district for his insight and he suggested that aged copper pipes usually have mineral/salt deposits on them. This "skin" prevents copper from being leeched into the water within the piping. The new-home pipes haven't had time to build up this coating and, therefore, the aquariums suffered.
In dehumidifiers, there's no chance of building this coating. So, copper-sensitive species will always be at risk of suffering toxic effects from the reclaimed water in copper-coiled units.
