I have also changed the puffer to a lyretail anthias. And the tang is a scopas tang.
Twice a week with those fish. And that’s the Bare Minimum IMO. Again, take it in steps with 1-2 at a time because even though the previous tank was running for a month Thats still a risk. Aggression from a tang, trigger and puffer is hell in a box, do the most peaceful first then the tang (Again, what type of tang is it?) and the trigger at the same time and then finally get the puffer into QT.
4 fish changes nothing, their bio load is the issue. 4 fish with VERY heavy bio loads in a tank that isn’t a big enough water volume to remove the nutrients is often bad.