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I've had a reappearing disease on my two corydoros aeneus for a while now. It is a small white spot on the dorsal fin where it meets the fish's back. I've tried various meds, none of which work, although I have not used parasitic meds except ick meds. They always go away after awhile, but I want to identify and eradicate this malady if I can. My ph used to be around 6.4, but I've now moved it to 7.0, and have been keeping it there. This does not affect any of my other fish (2 clown loaches, 3 neon tetras, one CAE, one swordtail, and one gourami). The cories are in a 20 gallon, but will be moved to a 55 gallon that I had finished cycling and put all the other fish in. I was hoping to keep the cories in the 20 for a while and try to eradicate this illness. Another forum I frequent was not able to identify the illness. The fish do scrape once in awhile, so I think it is a parasite. Thanks for your help or suggestions.

Sounds like ich (white spot). Treatments have been discussed here.
As a rule of thumb, the best way to treat ich is to increase the water temperature, coupled with an anti-ich medication.

no, it is not ick. It appears to be specific to cories and is only in one place -- the place I described. It does not affect any other fish nor do I need to medicate it for it to go away. It DOES NOT SPREAD to any other place on the cory. It is IN THE SAME PLACE ON BOTH CORIES. I want to find an answer so I can medicate it and make it go away for good, however I do not seem to need to -- because it will go away on its own. Still, it annoys the heck out of me.

In which case, a picture of the affliction would help. Several people on the forum have had problems with lymphocystis, a viral disease for which there is no effective treatment. That may be another possibility, although no one reported any scratching with lymphocystis.

I've wondered that, and was told in another forum that it was not lymphocystitis. However, for lack of another diagnosis -- maybe I will treat for it. Quick cure says it treats lyphocystitis ...I was hoping others would have had similar experiences, especially because this spot on both fish is very specific. It does not appear any where else but on that same spot on both fish. I do not have a camera, but maybe can borrow my bosses. Still, I doubt it can get that close up as both cories are quite small and tend to hide from me if I stare at them.

"Epistylus

Observations: Balls of fungus, usually located on dorsal and/or pectoral spines.

Conditions: Epistylus grows on gravel and moves onto fish with injured spines.

Fish Affected: Corydoras, Pimelodellas, any armored catfish, and iridescent sharks are especially prone, although it can occur in any fish with injured spines.

Cure: A combination of malachite green and formaldehyde, plus the actual removal of fungus balls to speed recovery." From http://www.fishmartinc.com

I think this comes closest to what I've observed with my Corys. I'm very dilligent with water changes and gravel vacs, but I did have an undergravel filter when this whole incident began. It also occurred in my 55 gallon tank with the same two cories for a time, but after the reduction in nitrates with more plants in the tank, I have not seen its recurrence in a few weeks, maybe even months (I'm losing track of time). Although in my case they would go away and medical treatments did not help (although I never tried malachite green -- mostly antibiotics and a bit of maricide.

Observations: Balls of fungus, usually located on dorsal and/or pectoral spines.

Just to be sure Epistylus are normally about 2mm in diameter and are slightly raised 1-2mm, is this what you are seeing.

If it does turn out to be I would personally let the fishes cure themselves.

Well, yes they were quite small and could have been raised. They looked like tiny white spots, much bigger than ick, one on each fish in the same place. 2mm sounds about right. They haven't come back in awhile, but it is nice to be able to at least point to Epistylus as to what it is. That's the closest anyway - but wish I could see a picture to be sure.