A healthy well established fish tank will have no ammonia in it, the bacteria will almost instantly turn the ammonia into nitrites and then nitrates both great sources of nitrogen for plants. So yes it is safe to use fish tank water on vegging plants I am going to start using mine more often for my veg plants. You just want to be sure the Ph is ok as said. Years back when I was more into freshwater tanks (into saltwater reef tanks now) I had a 55g tank with several chilids and live plants, I never had to do anything for the live plants and they thrived. There was always over 20ppm of nitrogen in the water if not more, these where fat healthy fish that ate well.