You can try API API Pimafix or Melafix or Furan-2. But those drugs killing
bacteria or fungi will most likely also kill your beneficial bacteria in the
tank.
I actually don't recommend use these drugs unless you set up a quarantine
tank for your sick fish. You can actually use Seachem Garlic Guard to soak
your fish food to feed them if they are still eating. It will boost up their
immune system to fight the disease.
On the other hand, your tank is overstocked, and I doubt you have a cycled
tank at this point. Have you checked the ammonia,nitrite, and nitrate yet?
At this time, you should change the water frequently: 20-30% every time with
water conditioner (Seachem prime or Tetra AquaSafe plus). The water
conditioner will detoxify ammonia and nitrite to protect your fish for up to
24 hours. You also can add a small amount of nitrifying bacteria (Seachem
stability or Tetra safestart) each time after the water change.
I highly recommend you to test the water quality in the tank.