I have a 125g FOWLR. Its occupants are a Snowflake Eel, Lunare Wrasse, Naso Tang and a Harlequin Tusk. The Harlequin and Naso are showing signs of Ich. I know I need to set up a QT, but what minimum size tank will I need. Do I have to all my fish in the QT. I still have to buy the tank and filter. My concern is how to quickly establish biological filtration.