Sodium nitrate is slightly toxic to fish, but only in quite high concentrations.
You'd have to pour Fiesta by the gallon into a pretty small pond to raise the nitrate concentration to dangerous levels.
NTA is toxic. The Sigma Aldrich MSDS for NTA
shows an LC-50 as low as 100mg/l over 96h exposure (so it looks like this was indeed tested against fish in other contexts, but other LC-50 numbers are as much as 4x higher; I'm just using the lowest one to prove the point beyond question).
Assuming a pond size of 50 gallons (ludicrously small), you'd need 19g of NTA (directly applied to the pond water) to get to this dose.
Back to the Fiesta MSDS, they say NTA is 0.5% to 0.8% of the herbicide, so you would need a little over a cup of Fiesta poured directly into a 50 gallon pond to reach this level. And runoff is NOT the same thing. NTA rapidly breaks down into much less toxic products and also binds to things in the environment, so I'm not sure how many gallons of Fiesta you'd have to pour around the perimeter of the pond for runoff to cause harm, but it would certainly be a lot, and overspray being an issue is just ridiculous.
The iron salts themselves range from nontoxic to slighly toxic to fish. Here's one reference to that.
But ponds frequently get fertilizer runoff (it's still bad for them, but that's another story), without the iron salts used in many fertilizers killing fish. In fact, I've seen chelated iron salts sold as an aquarium and pond supplement to help green up your underwater plants.
Landrus2 said to "Find out if there were any aerial spray for the West Nile virus at around the same time you sprayed."
and he's absolutely right. Pyrethroids used for this can be VERY toxic to fish, at seriously low concentrations.
Thanks very much for this! Yes, I did have the sheet and also knew this info (although not as well as I do now) which is why I was so confused by the customer's claim. Our spray was minimal. Will be interested to hear what the manufacturer's say. Thanks again for this Rlitman. Great work.