A few simple rules to follow regarding fish health;
Identity: Familiarity with its natural environment? Where was it collected from?
Behavioral: Passive? Reclusive? Predatory? Nocturnal? Energetic?
Physical: High/low disease tolerance? Sensitivity to changes in environment? Medication Sensitivity?
Water Sample: NH3
? pH? Salinity? Hardness? Temperature?
Disease Control: Physical/Behavioral abnormalities? External parasites? Internal parasites (regional)? Bacterial infections?
System: Flow-through? Pre-treatments? Other inhabitant degradation?
Treatments: Do you have a basic plan? Familiarity with medications? Adverse reaction control? Familiarity with dosages? Sedation techniques? Biopsy techniques?
These are just some basic questions you can ask yourself before purchasing a fish or even invertebrate. You do not have to be a doctor or vet. to maintain efficient disease prognosis.