Not negative, but positive.

I have couple freshwater tanks that are setup with high lights, CO2 injection and strict fert program. Since I dose macro and micro nutrients myself I know whats in it. I change 50% of water thats in my 45g weekly so I use it to water everything. From my house plants to everything outside and sometimes even my lawn gets some. You shouldnt have anything to worry about and that water shouldnt be high in nitrogen at all. There is nitrogen present I'm sure but if the fish are alive then its not as high as you think. Most fish tanks run with Nitrate(NO3)

Here is a link that might be useful: My garden

I use my aquarium water for my plants too. The nice thing is it doesn't have all the chlorine in it since I use a water conditioner. It also has some organic waste for the soil. While it doesn't have a huge impact on plant growth, it doesn't hurt either.

Alright, looks like aquarium water is getting used for watering, with regular fertilizer applied if/as needed. Thanks all!