Recycled fish water contains nitrogen which is one of the nutrients
plants use. Sounds like a good natural fertilizer, right?
Well, it can be but as the concentration of nitrogen in fish water depends on a number of factors it's hard to know if you're giving a
plant the right amount. Remember, too, that nitrogen is only one nutrient that
plants need, the other two major ones being phosphorous and potassium along with various trace elements. Sometimes too much of something is worse than none at all. So if you do decide to fertilize with fish water be cautious - use small amounts and watch carefully how your
plants react.