I have a small commet goldfish, about 2 years old that has a really stubborn case of a cottony fungus mainly on the tail, but also on the fins occassionally. Also, the tail periodically splits and then mends. This is all new in the last couple of months. The course of treatment I've followed is the one dictated on the back of the Melafix bottle I have. I have treated the fish at least three times at this point. The first time I treated it for a week, then he was fine for a week, then it was back. So, I treated it for another week--again, fine for a week, then it came back. Then I treated it for two weeks--fine for a week, then it came back. I just started another round of treatment again today. In addition to treatment, I have conducted 25% water changes every week to week and a half, and I take out the carbon filter for the durration of the treatment. The fish is in a 5 gallon tank with one other small shubunkin and a tiny plecostomus. The amonia levels are fine (I have a live amonia meter in the tank), I vacuum the gravel weekly, the temperature is maintained at about 75 degrees with a small heater, and I have live plants. In addition, the filter, when it's in, has activated charcoal and an amonia neutralizing material in it. Finally, I have regularly cleaned the filter case and the inside of the hood so that infected water dropplets are less likely to drop back into the treated water. The other fish are unaffected by the cottony fungus and the splitting fin, so this seems to be a stubborn infection just in the comment. What else can/should I do to clear this up for the fish?

Thank you!

The only thing that will clear this stubborn fungus permanently is a tank upgrade.

A 5 gallon tank isn't really even enough to house one fish let alone 3 however small. A comet needs 20 gallons, the pleco needs 20 gallons and shubs are long sleek fish that do best with 20. In smaller spaces the bacteria levels become lethal and I am very surprised you can keep on top of ammonia levels with a plec that needs constant feeding

At kokos and most goldie sites 10 gallons per fish is the recommended space allocation per fancy fish, 20 for each singletail faster moving fish- and anything less will -eventually and sadly in most cases fast and soon -make the fish sick. The cheapest health plan in the long run is a tank upgrage. Adding meds as you have found is a band aid that is not tackling the underlying problem. I suggest at least 50 gallons, more is always better

With 3 fish and plants, there is too much competition for oxygen and nutrients.

Please don't use those meters...they don't work. Go out and buy an API master test kit, and test your PH/Ammonia/Nitrite/Nitrates, and report back. I think you are going to be surprised what you see.

Ditto all the above.
Plus one other thing; plecos are not recommended tanks mates for goldfish as they quickly develop a taste for their slimecoat.
Many GF tails on this site have been chomped by plecos, so moving him into another space would be preferable.

Thanks for the suggestions--I have an empty small tank that I could set up for the pleco. He could become my office pet, I suppose. I could move one of the two plants in with him. And I'll look into the API test kit you suggest. I have my current little menagerie as a result of some apparently bad advice from the fish folks I asked questions of--they seemed quite knowledgeable at the time... I was having trouble with algae, so they recommended a pleco and then plants for him to hide in. I already had the two little gold fish (both two years old) because I was told that one fish would be lonely and depressed, so I got a pair of "friends". And throughout this whole process I told them I was dealing with a 5 gal tank 'cause I thought that this was an awful lot for a little tank, but they said it was fine. It seems that it isn't! I don't have room (or money--I'm a starving student) for a bigger tank at this point. Should I find a new home for my healthy fish and nurse the sick one along?

I totally sympathise. We have all been told terrible untruths by fishstores
You would be best to rehome the plec certainly if you can. Sometimes stores will take them back..Could you get hold of a rubbermaid container for the other 2 and fix up the same filter? These two are going to grow and it will be impossible to keep them healthy a long time in a 5 gallon. It will be back to back problems once the first starts and would commit you to 80% water changes daily to get bacteria count decent. And even then the fish will be stunted which causes a farther set of disease issues-swimbladder and flipover for example.

If you could fix up a rubbermaid container-they can look very nice and are not so expenive, you can transfer your cycle and will get far less algae because of the space. Besides, some algae is a good thing. It keeps nitrates down and the fish can graze on it for extra nutrition

Thx for posting back. We all didn't mean to scare you off! Most of us arrived here in the same shoes.

Thanks--I'll call the petstore and see if they'll take the pleco back (his name is Fidel because he kinda looks like Castro). Then, in the short term, I'll set up the other small tank I have to nurse the sick fish (named Squeek) and then clear out most of the water from the current tank to reset things for the shubunkin (named Schoobie Shlobotnik, the Shununkin from Sheboygan--had to work the word Shubunkin in there somehow!). Then I'll see about what can be accomplished re: an overall bigger tank or rubber maid--besides money, a secondary issue is space and a tertiary issue is keeping my cats from eating the fish, so an open rubbermaid might not be ideal. Perhaps I'll have to find new homes for the fish when all is said and done because I want what's best for them.