What are your water parameters?
Ammonia
NitrIte and
NitrAte
pH
TDS
KH GH
water temperature
What size is the tank
What fish are in it?
What is the filter?
How long has the tank been set up?
How long have these fish which are getting ill, mainly the Gourami and then the Cory, been in the tank?
What is your tank maintenance like? How often and how much water is changed?
Are you familiar with the water nitrification cycle? The Aquarium Advice article linked in my signature gives this critical information and other tank fish and water keeping helpful information.
Are there any teeny worms (flukes) moving around the gills, eyes, mouth, anal vent?
Any white, mottled or dark grey/black teeny things / Fish lice, creeping around on it's body?
Are the exact same spots still in the exact same places on the fish since you first saw them?
If so, check into Lymphocystis.
https://en.wikipedia.org/wiki/Lymphocystis
~~~~~~~~~~~~~~~
Water quality is #1 most important thing for the fish issues in the aquarium.
Make a couple pwc's one after the other, 25% each time. Then check the water quality. Then if not perfect, do another water change in an hour or 2 of about 40%.
Check the water quality again until you have
Ammonia 0 or less than .25
Nitrite 0
Nitrates under 40 is best
Figuring out if your pH, TDS, GH/KH are in a good range for your fish inhabitants. If not,often after keeping the best water quality can be enough for them to adapt. Sometimes it is too different and then there are other things to explore.
https://www.aquariumadvice.com/fish-disease-index-for-diagnosis-and-treatment/
Aquarium Salt dip
https://www.aquariumcoop.com/blogs/aquarium/aquarium-salt-for-sick-fish
Epsom salt (which is not really salt but magnesium sulfate) therapy
https://www.aquariumadvice.com/forums/f17/using-epsom-salt-in-fish-therapy-374191.html#post3548383