Filtration for a 120 gal?

I have my 120 going, planted, with about 20 fish. 10 Cory cats, 3 Dwarf Gouramis, 5 SAE, a few cherry shrimp. I have a Rena XP3 canister filter. Today I came home and the water looks a bit cloudy and one of my Cory cats looks like it has a fungal infection on its fin. It looks like a cottony growth. I looked it up and it says its due to poor water quality. I have been doing very frequent water changes.
Could it be that my filter isn't good enough?
It says its for aquariums up to 175 gallons.

I am going to have to move fish around tomorrow and empty my 10 gallon to move this guy into and treat, or should i treat the whole tank?

This is my first sick fish and I have had these guys about a month. Moved them into the big tank last week.

Any suggestions? I will do a water change tonight.

The tank has been running for about 5 weeks. I do a 40-50% water change at least once a week. I feed them every other day. I always tend to overfeed, haven't found the right amount yet, eyes bigger than mouth.

i just cleaned the filter a week ago too. If i get another filter i dont know where it would go, the intake and outtake bars take up so much room and make such a current. Should I isolate the little guy or traet the whole tank as a precautionary measure?

i check my parameters every other day and i am due to do it tonight. as of last time i had no ammonia, nitrite or nitrate. I just got some fungal medicine for my fish. I have one fish in the 10 gallon tank i would use for quarantine. do you think it would be okay to leave it in there and just treat both fish?