Desktop lamp with daylight bulb (or 2 of them).
I'm buying damaged LPS cheap on a regular basis, most are OK, but every trachyphyllia/welsophillia dies, same tissue necrosis. And had a tank crash recently - again, several corals dieing and all heaters become tied in another tanks and containers.
Dead tissue fouls water quickly (20g and 4.5g), and other corals react visibly: if you can - better to remove in separate container, if can't - siphon out the dead tissue, by tubing or turkey baster. Blowing it out under powerhead, as for a tree sponge, is not an option, too much of it.
Then - as curing live rock: main die-off could be swished in the separate, say 2-3L container, clean wash in the second bowl and move back to the tank. Same the next day, in 3-4 days all will be finished.
More water changes, adding Seachem Prime daily - you know the routine.
3 dieing trachyphillias, following this procedure, didn't crashed 4.5g container with other open brains: scolymia and cynarina. They were not happy, but recovered in a few days. No skimmer, HOB filter only, filter floss and carbon were changed every day.