If your thinking that 3 fish in a 20 for 6 weeks is problem, it's not. They need minimal light, and HOB
filter w/wo biowheel will do just fine.
When I had an outbreak in 125 I went out and bought a 30 gal
tank and hob
filter. I put 8 fish in that 30 and several pieces of pvc
of different sizes for hiding places. I did 2x daily pwc
for a week till the filter got going, then a pwc
every other day for the next 6 weeks to maintain the water quality. I also had to remove almost all of the LR
so I could catch the fish to put into the QT, but that gave me the opportunity to re-aquascape.
Yes I have a full time job and worked some late nights and some weekends during that period. I wanted to get rid of the Ich in my tank, and be relatively sure I would not get another outbreak. This is a lot of work, but this also an expensive hobby, especially if you always have to replace fish.