A molecular biologist working for a government-run fishery in Vietnam

A molecular biologist working for a government-run fishery in Vietnam is interested in controlling Pfisteria in fish farms. Pfisteria produces toxins that stun the fish and then causes the skin to slough off, allowing the dinoflagellates to dine on the tissues of the fish. He needs to develop a treatment that kills Pfisteria without harming the fish or the beneficial green algae that serve as food for the young fish. What strategy should the biologist consider for developing a selective treatment?