Define the project
Record the species, life stage, water conditions, process and outcome of concern. “Fish welfare” names a field; a useful project needs a more precise boundary.
Ask what is measured
Distinguish measures in the animal from measures of the environment. Explain who acts on the information and what comparison would support a claim of improvement.
Keep the scope visible
This is an investigation framework. It does not recommend a method, classify an operator or transfer evidence between species. A site-specific conclusion requires competent specialist assessment.