The cause of this problem is a 20 gallon tank with an oscar 2 plecs, and goldfish, all of whom are very messy fish. That oscar will need a 75 gallon when it is full grown, that is for the oscar and nothing more. Goldfish need 20 gallons for the first fish, 10 additional gallons for the second, as they get big. More than likely the plecs are common plecs, they will get over 12", and have the potential to reach 24". The oscar & plecs are tropical, the goldfish are cold water, and really won't like the warmer temperatures the other fish need.
What you are describing is columnaris, a bacterial disease which often looks fungal but is not. Ick is tiny white dots, like the fish was sprinkled with salt. The white feces is probably caused by an internal protizoan, this means you have two medical situations going on which need to be treated differently.
I hate having to tell people this, but unless you get a bigger tank, as well as rehome the goldfish, those fish are goners. I wouldn't keep an oscar and two common plecs in anything less than 100 gallons, with double the normal filtration. If those are a smaller species of plec such as bristlenose they will get eaten by the oscar, I've had that happen. With good care an oscar can live over 10 years, common plecs over 20 years.
Petco is having their $1 per gallon tank sale starting next weekend, I would get a 55 for the oscar and plecs, for now. I would rehome the goldfish, unless you want to pick up a second 20 gallon & keep one in each 20.
I would treat the bacterial problem first. Maracyn & Maracyn II are a couple of broad based antibiotics that are commonly available, and are usually pretty good with columnaris. Treat all the fish in the 20 gallon, keeping the temperature around 76F.
I would then put all the fish but the oscar in the 55 gallon. Read up on tank cycling, you could use about 2/3 of the media from the 20 gallon in the 55 gallon filter. I'll give you my usual copy & paste for internal parasites;
This is what I do for angels, you may want to look into the maximum temp your fish can handle. Metro was originally designed for humans, the closer you can get the tank to 98.6F the better.
Put the angel in a quar tank, I usually use a 10 gallon. Increase the temp over a day or so to 90-94F. Treat daily with 40 mg/gallon metronidazole, with 50% water changes daily. If the fish isn't eating, don't feed for the first 3 days. After 3 days, get some frozen brine shrimp. Take a portion about the size of a few match heads, and sprinkle on some metro. You have to eyeball this one, make the shrimp look kind of like a tiny powdered donut. Once it thaws, mix it in & feed. Sometimes they still don't eat for a few days, but once they start to mouth the food & spit it out, the meds seem to get in their system real good. Continue medicating the water & food for 7-10 days.
Metronidazole is sold as flagyl, het-a-mit, and a few other names I can't recall offhand, used to treat hexamita & hole in the head. They usually suggest 20mg/gallon, this often isn't enough. Metro deteriorates in 8 hours, so you could do twice daily water changes. I've done this on the weekends when I have time.
I've also added epsom salt while using metro, this helps with any constipation issues, cleans out the digestive tract. I start with 1 teaspoon per 10 gallons of water, increasing by 1 teaspoon daily for 2 more days to bring it up to 3 teaspoons per 10 gallons. You will have to adjust how much you use when doing water changes. You may want to give this a try.
I treated my oscar for this when he was younger, I had the temp up to about 90F. He's about 10" now, and still growing.
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