Couple things.... Yes it is in your tank, it is in the gravel so Vacuum Vacuum Vacuum
Then, treat with Voogle , Kenamyacin ( spelling?)
or lastly Melafix.
I believe you can only get Kenamycin from a vet. You néed a broad spectrum anti fungal antibacterial .
Also temp in tank below 76 or above 87 to kill buggers
One of my angels got that and died despite meds