I have two 10 gallon fishtanks (well, one's a salamander tank, only about 2-3 gallons of water in it) that, of course, have to have the water changed regularily. My tanks are as close to natural ecosystem as I could get it; I use very few chemicals, relying on a good bacteria colony for Ammonia breakdown instead.
Last year was my first year gardening and I decided to conserve water by using the water I change out of the tanks to water and fertilize my garden. I don't know if this is actually a good idea or not, but the results were pretty good! I used this water about 90% of the time (when it got too warm over a short period of time I had to resort to the hose) and only added other fertlizer to do my tomatoes now and then. The only other thing to be careful of was to make sure I washed by 'edibles' before consumption which I would do from the store anyway.
Again, I don't know if this is normally recommended but it worked great for me and helped keep my water consumption down and my garden looking great (expecially for a newbie)!