How do you measure 400 mg in spoons?

A density of about 5 grams comes in a teaspoon and as you require 400 mg which means you need 0.4 grams which is equal to 12th part of a teaspoon. If I've done my math correctly, that's 1/4 teaspoon per 3 gallons water. This mixture of powder tetracycline is used in cold water for treating hens with CRD. Please follow the dosing instructions on your package; I'm not a veterinarian.