Fish water is very high in nitrogen and nitrites, it might be good, but you will have no way of knowing how much nutes your adding to your plants cause tank waters changes from tank to tank, nothing is set in stone. It all depends on how many fish you have, how often you change their water, ect... so in short don't do it, if you were looking for a cheap nute it will not work, suck it up and buy shit made for plants... On the other hand, i know the plants outside (not marijuana) have exploded with growth since i have been throwing my old fish water on them, but nothing i would do with my Babies