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When I read this node title, I thought to myself: well, that's pretty obvious. And I also anticipated someone's well constructed (probably) description of a funny home experiment with mary jane and a goldfish. To my surprise, nothing! Well, this begs for rectification:

Almost all plants will grow better if watered with the water from a fish tank, since fish excrete in good supply the two most important nutrients for plant growth: phosphorus and nitrogen. Thus, if you compare two plants, one which is unfertilized and the other watered with the water from a fish tank, it is only logical that the latter grow better, since it is essentially being fertilized. In fact, I would bet that such a plant would even have an advantage over a fertilized plant, given that chemical fertilizer is presented in inorganic form, while fish excrete organic phosphorus and nitrogen molecules, more readily assimilated into plant tissues. Of course, the relative dose plays a role, but I would wager that fish tank water is, mol for mol, the better fertilizer.

I should also say that my hydroponic horticultural experience is limited, so I shouldn't pronounce myself with great authority on the subject of proper cannabis production. So here's the caveat: If you water your plant with the water from a fish tank, each toke may carry with it a wonderful aftertaste of fish poop.