I've had planted tanks extremely high in inorganic PO4 and inorganic NO3 with no effect on breeding performance. The mis-correlation occurs when we assume that organic forms of these components are toxic. If your tank is high in organic species of NO3 then the damage has already be done by what the NO3 started off as - typically ammonia or other forms of nitrogen bound in organic waste content such as proteins and so forth. The end product is NO3 but this is not what does the damage. The same can be said of PO4. If it's origins are organic, such as food, decaying vegetation, or feces, then it's release into the system is a result of Redox reactions which lower oxygen levels, for example, in order to breakdown the components. Organic PO4/NO3 are merely the smoking gun. This is why water changes are so important in a CO2 injected tank. Not to remove NO3/PO4, but to remove their detritus/waste product origins which are highly toxic. High CO2 equals high growth rates which generates high levels of organic waste. So it's as simple as keeping the tank scrupulously clean. Then you will see that it is not the NO3/PO4 levels that are relevant, but merely the reduction of organic waste from which they originate.
Inorganically dosed NO3/PO4 are greener and cleaner simply because they do not require breakdown and extraction from toxic components. They are immediately available, have little effect on Redox potential and have minimal toxicity. Inorganic salts added to the tank do however sharply increase the Conductivity (Total Dissolved Solids) and therefore, if the breeding fish are sensitive to TDS then this will be an issue, but again, it has nothing to do with toxicity.
Cheers,