I purchased some bait goldfish as starter fish for my 40 gallon tank.
They came down with ich. I used lemon essential oil and it worked! Six drops the first day and two drops every day for a week. It was gone in three days and hasn't returned even though I've added fish from my pond to the tank.
I'm thinking it made the water alkaline so if that's the case, be careful if you already have plants in your system. At the time all I had was berry and tomato cuttings along with some basil and oregano. They werent harmed.
I used therapeutic grade essential oil.
I hope this may help someone!