Generally, used irrigation ...

Generally, used irrigation water would have minimal residual nutrients unless the original users weren't careful about their budget.  The mix would be diverse in any case, with the final amounts in a storage tank depending on the consistency of the sources.  As a monitor it would be possible to select just the most common nutrient and measure the amounts in the collective vat periodically.  That would at least tell you if you have a real cause for concern or nothing much to worry about.

Preventing algal "blooms" regardless of nutrient content may be easier, as they require sunlight to prosper.  Simply depriving the solution from sunlight by using covered ponds or completely closed opaque vessels would prevent any algal bloom from occurring, even in the presence of sufficient nutrients.