the water in an ordinary freshwater fish tank becomes super-saturated with all manner of things plants find delicious. every drop of waater i took from my tank full of mollies and tetras went on my garden, and i never bought a drop of fertilizer. unfortunately i cant imagine any setup which might allow you to keep fish in a hydroponics system.
1) it takes a long time for fish to fertilize the water.
2) any plant tonics you put on your plants will get into the tank, and kill your fishies.
3) anything you put in the water for your fishies (ick medicine, a heating element, sea salt, algae eating fishies, etc) will be hazardous or lethal to your plant's roots.
4) FUCKING ALGAE
5) you wont be able to see your pretty fishies through the root system
6) fish tanks hate change. when you harvest your plants your fishtank might become toxic
7) if shit goes south and you need to scrub your system, shit will get complicated.
how high will your growroom have to be to accommodate vertical cannabis growth, and your fishtank?
9) youll never know exactly how nourished your plant's water is. this is no problem for ordinary plants in an ordinary soil garden, but cannabis cultivation is not like growing tomatoes on the patio.