Mystery Parasite

Hello everyone! I hope you can help me. I am a long-time fishkeeper with a startling new problem. I have somehow aquired a mysterious parasite that resembles ick but drops off everytime I go for a scraping.

It all started about a month and a half/two months ago. I noticed some white spots that visually resembled ick in my new planted tank (10,000K compact, flourite/eco-complete substrate). At first my rainbows showed spots. I isolated them and treated them for ick. As other fish began to show spots, I removed all fish from the tank into quarantine, upped the temperature to about 82 degrees, and left it empty for a week and a half. I treated the fish with malachite green, then acriflavine, and they were non-responsive. When I tried to remove a fish for a scraping, the parasite fell off! I treated with salt, and increased the temperature to 80 degrees in the quarantine too. When the spots had cleared up for several consecutive days, and the main tank was very empty, I assumed it was clear and put them back in the main tank. About two or three weeks passed by uneventfully, and I thought it was all over.

As of yesterday, it's back in a big way. I did my usual daily tank inspection to find the Rainbows reinfected. I had some sailfin mollies infected too (I breed these guys so I have hundreds of them, but only a few rouge babies I couldn't catch live in this tank among the plants). Stronger fish that live in the same tank (wrestling halfbeaks, shell-dweller brevis, pleco, etc) do not have a spot on them and do not even seem to notice.

I told a friend with a degree in zoo-style fishkeeping about it, and she lent me a book (it was several week ago, but I believe it was pathology of fish diseases). The only thing even remotely close was paramecium infestation, but that is not supposed to reach pandemic proportions! I thought, since it survived so long without a host, it might not be some larger parasite (the spots are very distinct, mostly on caudal and pectoral fins).

Here is the weird part: I have not lost a fish to the white spots yet! They do not shimmy, nor do their fins get tattered. Their epithelial and slime coats are intact, and their appitites are as good as ever. They act like they are not sick at all! I did lose a few fish to the arduous treatments, however.

I am willing to treat with formalin, if I must, but only in the quarantine tank. I am treating for external parasites now. I will continue to try to get a scraping to look at on a slide, maybe I will get lucky.

Can anyone help me?
Morg