Yikes, sorry for the loss of your fish but I hope you are not blaming the "caretaker".
I have never heard of any city adding anything to water that was toxic to fish aside from chlorine or chloramine.
People regularly add "algaecides" to their ponds with no adverse effects. Can you call the city and share with us what exactly they added and the dose?
Four inches in a 3000 gallon pond should not really be enough to be toxic, with untreated city water, unless the pond is very shallow.
As far as friendly fish and feeding goes this is a mystery.
Have you tried different food types and maybe not feeding so often?