"Gray water" should be fine for watering garden plants or making compost tea as mentioned in the articles you linked. With all the organic matter and humic sources found in compost tea, tap water usually isn't a problem anyway. For an aquarium that has the need for such soft water, distilled water is cheap enough. I pay like 60 cents a gallon for distilled water around here. I have an R/O unit as well but it's very wasteful... way more than a few gallons you might toss out from a dehumidifier. Rain water might be another option, depending on where and how you collect it. In other words, you might not want to collect it from your gutters or in high pollution areas.