We bought a 50 gallon aquarium in Japan. It is longer than most U.S. made fish tanks. I have goldfish in it that the kids enjoy. I've been taking water from it to water houseplants. I was recently wondering if I could just drop some floating aquaponics stuff on top of the water in there and start growing things. Whatcha think? I wouldn't pump the water out anywhere, I'd do it all in one.

Plants are a great way to clean the aquarium water but I would still recommend water changes occasionally. Duckweed is a good plant for goldfish. Under good light the stuff grows fast and the goldfish will eat it too. Be aware that if you have a Hang-On-Back style of filter, the duckweed will tend to clog it up as the waterfall effect tends to push the duckweed down into the intake.
Some plants don't like to have their roots left in water but I find that a good airstone keeps them happy.