During my work I am often asked to make diagnoses over the phone. But it is quite difficult to make a diagnosis without having seen the pond. While the customer’s precise description is often very helpful, a diagnosis with help of a microscope is more accurate. Treatment and cause diagnosis is a lot faster and a smear can be used to treat the algae problem more quickly with the right compound.
Of course your input about fish stock, pond equipment, pond conditions, water values, location, filling method etc. is also very helpful. You can buy very good microscopes relatively cheaply in specialist shops. Otherwise you can ask your koi dealers whether they can make a diagnosis by microscope, or ask at your vet’s.