Hi Misky99
If it were me, I would discard the substrate and disinfect the tank using a diluted bleach solution. A tank disinfected with bleached must be rinsed, rinsed and rinsed again very thoroughly...to be on the safe side after disinfecting, a dechlorinator chemical can also be used in the tank. I would also disinfect any water dishes, etc in the tank....and discard or try to disinfect any live plants you may have. As your vet diagnosed an infection and the antibiotic treatment initially was effective, I would disinfect.
If the one tiger still has the black spots, move it to a hospital tank using white paper toweling that is moistened and changed daily...with crumpled moistened paper towels as hides. I would not place the two animals together until all signs of infection are gone and a few weeks have passed to assure that is the case.
Regarding a substrate using organic topsoil alone or mixed with coco-fiber is really the best. You don't want peat moss or sphagnum peat due to acidity. Pine and cedar bark/mulch must be avoided due to toxic volatile compounds. Other types of mulch, if course, can be sharp and may pose a hazard when the tiger lunges for food. Gravel should not be used for the same reason. Reptiles vs caudates have different tolerances including the need for light/heat. I'm not sure where you landed re stopping the use of a light bulb or UV light, but I would not use either. There is a chance that using these set up the situation that resulted in the infection as tigers have fragile skin.
Please keep us posted and best of luck.