Here's the link specifically for M blue https://www.reef2reef.com/threads/methylene-blue.257467/ all you need is a 5g bucket or two. If you have a spare powerhead I would recommend you aerate the bath water heavily for about 30 minutes before doing the bath.

. You said that the fish have been in QT for 3 weeks but you've had them for 2 months and you are treating for ich, yes? So were they in your DT when you suspected them of having ich?

