Someone asked that question to the Fish Vet - Dr. Richmond Loh in Australia. The answer may surprise you!
We can’t think of anything that humans can eat, that is directly toxic to fishes.
Two things we’ve identified as being potentially a problem are:
1. No sugary foods because the carbon in sugars can lead to bacterial blooms, leading to oxygen depletion and fish mortalities.
2. No oily or fatty foods because it can create an oily film that will prevent gas diffusion at the water surface, and fish will suffocate. Plus it’s not good for their long-term health.
Dr Richmond Loh DipProjMgt, BSc, BVMS, MPhil (Pathology), MANZCVS (Aquatics & Pathobiology), CertAqV, CMAVA, NATA Signatory.
Aquatic Veterinarian & Veterinary Pathologist