What are the effects of "dangerous levels" of nutrients?
most nutrients useable by plants take a lot to be toxic to fish
one being copper of course but using common fertilizers its hard to overdose that
some people say over 20 ppm nitrates its dangerous. i've had fish at 200 ppm with no ill effects. and phosphates at 15 with no ill effects
the most common effects of dangerous levels though, are poor respiration, and low immune system.