I'd be happy using un-medicated aquarium water on a garden. I'd be concerned about using it for houseplants because you can concentrate the minerals in the water over time.
The water evaporates of the tank, and you top it up, and thus, you concentrate the water minerals. This is a long term concern, potted citrus, ect. You do this on pots that are only going to using that soil for a few years, no worries.