Correct, sorry if I wasn’t really clear. My DT is 55g treated 2 clown prazi+metroBy the way - I'm confused. So forgive me - since there is a copper level problem. In your OP you say its a '20L' tank - but above you say its 15 gallons filled? Again maybe I'm misunderstanding - and I know you have 2 tanks - so if I'm misunderstanding - apologies.
My QT is a 20L that was filled at about 15G maybe a bit more to allow room if I needed to dilute copper concentration without having to remove water. This one is treating the tang