Black Ich is actually caused by an infestation of tiny turbellarian flatworms.
Although it's not as dangerous to the fish as crypto it still as an incubation time after dropping off the fish.So I would imagine that anything like substrate or LR moved to the new tank quite possibly could have the parasite ready to hatch.
1) I'd remove the tang to a treatment tank.
2) Treat the fish with formalin along with the other fish that were in that system.
3) While this is happening I would set up the new 125,new substrate with a cup or two of old.Or if you wish,use the old substrate from the 55 gallon along with new.
4) Take everything that you are going to use from the 55 and put it in the 125 gallon,LR..ect..ect.Leave most of the water.
5) After treating the fish for the recommended time period with formalin,i'd move the fish back into the 55 gallon.
6) No substrate/rock,just PVC pipes for the fish to hide behind.A heater,filter and PHs on this tank.You can put any inverts(crab,snails,shrimp) in the 125 gallon if you have well cycled rock and have no worry of a cycle.
7)Observe the fish while they're in the 55 gallon and leave the new 125 empty of fish for the recommended tome period for the parasite to go through it's cycle.
8) Move the fish to the 125 after parasite dies.
9) SAVE the 55 for a future QT tank.