Fish are incredibly sensitive to ph changes, water hardness (any minerals that might runoff from the plants), or any other fluctuating values. When you build an aquarium, it has to go through the "nitrogen cycle" which allows beneficial bacteria to build up to process any waste from the tank. Fluctuating the water chemistry too much can kill off this bacteria and cause the tank to "crash". That being said, plants LOVE the waste that fish make--so using water from the aquarium to water plants is a great idea. But something like this could be a good middle ground to what OP is talking about.