Looks like 18 drops raised your tank .11 ppm. In theory, you would need to add 36 more drops to hit 0.50. However, copper is weird, it binds with calcium in the water at first, and less so later on, so you will probably not need to add that much. Try adding another 18 drops and let it mix and retest.I just tested cooper again and it is .28 now. I added 18 drops on the second dose. I have a 30 gallon hang on filter on the back with bio bale in it and a air foam bubble filter. How much do you think I need to add?
Jay