How long do you leave a tank fishless for ich to die off? Let's just get into the real reason im asking... I'm picking up a sailfin tang from a friend tomorrow (not my idea, I just don't want to see it go to the awful lfs near us) and I only have a 20g qt tank. This is not a small fish, maybe 6-7". My display tank has been fishless for almost 4 weeks and I need to know when it is safe to put the sailfin into my 120. ps-- I know the tang can't stay in the 120, but again, the lfs is terrible. they have an 8" black hawaiin trigger in a 10g tank. That is a $300 fish!!! pss-- I may be picking up a running 60g next weekend, so I could move the fish there to qt it properly if I need to.