The practice of Veterinary Medicine “without a license” would have you diagnosing diseases and making prescriptions for fish*. To avoid doing that, you can make an assessment of the pond and fish and note ‘what you found’ without actually referring to a parasite or condition as a diagnosis.
Identifying the presence of a parasite isn’t a diagnosis. Identify a parasite or condition without making a diagnosis like a veterinarian. Here’s a document that steps you through the many assessments you would do well to annotate WITHOUT making an actual diagnosis.
*Because you can diagnose and give shots to dogs and cats without a license all day long and not get punished.