As I said before, I would not be surprised if it were a toxin due to the acute onset of symptoms and the fact that multiple fish were affected but I question the phosphorus theory. Phosphorus comes in many forms but the type found in tanks (due to decomposing organic matter and such) is not toxic to fish even at pretty high levels - of course anything is toxic at a high enough level but if that much phosphorus is in your water I would think there would be a public health alert. Of course there could be fertiliser run off, were they referring to organophosphate poisoning? If so, the aquarium phosphate absorbers will not work well if at all. It might be worth searching out a better water source.
Anyway, I am glad your fish are doing better.