I can't see the picture here at work, but if it is ich then you have the following options
To treat, remove all fish from the DT and do one of these Tank transfer method(TTM), copper, chloroquin phosphate( never used) and finally hyposalinity. In addition leave the DT fishless or fallow for 72 days.
The other option is to do nothing try to keep good water quality and feed well and hope the fish weather the ich and reinfections. Ich will still be in your DT so each additional addition can come down with it.
good luck just my 2 cents