So sorry, Sushi.
You won't get far dealing with the city. And I would consider an outside laboratory - if the city inspects its own employees, you know they are going to decide that they did nothing wrong. We had a city police horse knock down and trample a woman and kick out many teeth, but the city won't do a danged thing about it. Many municipalities have rules that they cannot be sued!
But you CAN do SOMEthing. Call the EPA (U.S. Environmental Protection Agency), the state bureau regulating pesticides (pest technicians must be trained and certified in most states, and there are laws regarding what they can apply, how much, etc.), and try your state Fish & Wildlife Dept, or Department of Parks & Wildlife or whatever name they use. City and State Department of Health & Human Services is another. The Humane Society or some wildlife rehab organization could direct you, too.
I doubt anyone will replace your fish, or even care very much, sad to say, but you can make life very miserable for the technician and his company.
Don't give up. Don't let them hurt another pond/family.