If you're helping your fish fight an illness, I would do 5-6 gal (50-60%) at a time daily or every other day - clean water can help.
Unfortunately I have no experience treating for mouth fungus though I have heard about "cotton mouth" - can't advise on meds - hopefully someone else will chime in here.
thanks unfortunately he looked to far gone so we put him out of his misery now i just want to prevent the other fish he was housed with of getting it to. he was by far my favorite so i am really sad now.
i am going to go get the meds that was mentioned here and watch my other fish like a hawk.