I have also heard a lot of people watering thair plants with the water from the fish tank.
I however have never tried it but I have a saltwater aquarium!
There are plenty of fish emulsions that claim to be great substinance for plant growth out there. I have looked into it and from what I have heard, people get great results out of the emulsions. Also, there are even some people on this website that have made "soups" out of fish parts and from the pictures they have provided, it looks pretty great!
I think with anything you need to check the balance of the NPK content in the fish water. Those are the 3 main elements that plants love and are what is most vital to plant developement.
I think if you want to save some money and decide to NOT buy nutrients then Aquarium water might help but it is much easier to add your own.