You need to get all ypir fish into a Quarantine tank. Get a 20G high, and Aqueon Silent Flow Filter. And a 100W heater.
Treat all the fish with Prazipro. I would follow up with treatment for Ich just to be safe. But the pics you have are not ich. Ich is small white raised bumps
.. Smaller than what you have.. And raised.
To treat for Ich, do not use any carbon in your filter. To use the Cupramine... Gonna need a Hannah checker to make sure the copper is the right concentration. Also treat with hyposalintiy (low salinity) - keep the water at 1.017 instead of 1.022
Takes about 2 months sometime 4 months to be sure the fish is cured.
Cant have any inverts or coral or live rock in there if using any meds... Need a separate Quarantine Tank. It will kill your live rock, destroy it forever with poisonous copper, kill you corals, kill your live sand, AND continue the silocone in your DT and will leach poisonous copper forever. So need a 20G for fish Quarantine only.
If you are new to this, please listen... You MUST quarantine everything before going into a DT. If you do not, you are going to spend a Lot of money of livestock and it WILL all die of SOMETHING that gets in there.
Do not buy live rock from the LFS... Buy dry, cure it yourself.
Make sure to dip any corals using CoralRx --- and corals need a separate QT.
QT inverts too - at least a couple days... I have had hitchikers come in on Snails! Ich eggs can be on snail shells - brush them with a toothbrush.
I cured 7 fish of Ich using Cupramine AND hyposalinty combined... Nees both. Doing one without the other is only half as likely to work. The icy returned 3 times... Had to treat for 5 cycles of Cupramine. Fish were in quarantine for months.

But - I never lost a fish.
Fish of Hex on Youtube has a great video for hyposalinty and ich.
Pics look like Brook. Your other wish probably have it now - and all nees to be put in QT and treated.
2 Quarantine tanks: One for fish, one for corals and inverts.
Quarantine
Quarantine
Quarantine
I can't stress it enough. I have personally lost thousands on a tank that crashed from every thing you can imagine: Marine Velvet, Brook, Icy, Internal and External parasites. And even parasitic "bugs" that came in on live rock from Figi. Been where you are... I leaned the hard and costly way.
Get that fish into QT. Petco has ths best deals for such a fiasco.
Best of luck. Quarantine!