Moving yellow spots on fish

[Crystal]

I had 30ish 1" BN plecos in a quarantine tank, some started scratching so I added salt. The next day many sported a few spots which resembled ich, I increased salt and also added some coppersafe (I have encountered ich many times and rarely have issues defeating it, except with the more sensitive tetras). By that evening all fish had numerous spots and some were already dead. Next morning most were dead. Did a 50% water change and adjusted salt and coppersafe levels to match the tank water before adding. By evening all fish were dead.

I am completely stumped and as I stared at one recently dead fish, I realized the spots were actually yellow - I had simply thought that the yellow skin of the albino BNs had made the white of the ich hard to see. Then as I was watching, one of the spots starting swimming away with a jerky motion... The spots are a little bit bigger than the ich I have seen before, and upon closer inspection, there were many of the "spots" swimming around, and even more clinging to the glass. They appeared to be completely unaffected by the levels of salt and coppersafe in water.

I have never encountered a parasite (outside saltwater), that appears and kills all fish within 3 days. Anyone have any idea what this is? I have an angel fish in a different tank also showing these same "spots" and I really do not want to loose this pretty angel!

[jvision]

Were these Plecos wild-caught? I've seen a few different parasites come in on WC fish, and have had success treating by adding salt at the higher end of the ich treatment (1 tsp/gal - normally I'd add 1tbs/5gal) and add Melafix as per directions.

[gobies et al]

I know this is controversial but... many pleco's have a poor tolerance of salt and metals (copper). I don't know what the parasite you are dealing with is, obviously not ich if you can see it moving.

This site: http://www.plecofanatics.com/forum/archive/index.php?t-32279.html

states "Those fishes are, I would like to again point out, barely armored against poor water quality and are susceptible to "organic pollution". Maintain water quality as hign as you can. On imported specimens, internal worms and gill flukes may result in hard troubles. During quarantine, feed your new fellow(s) a medicated (anti-parasitic) food for at least a week. Otherwise, you can rely on metranidazole/flagyl (that’s what I did, btw!). Captive-bred specimens are, of course, almost always gill and internal worm free. Infestations of ich – please not! - can be handled with green malachite (no personal experience at all, knock on wood!). Do not use too much salt, which is quite common when dealing with cichlids, or metals (copper) on catfishes since they could die from these treatments more readily than the apparent cause you're treating."

[Crystal]

I didn't use as much salt/copper as I normally do as these were 1" fry, I used about 3/4 of usual the dose. They weren't wild caught, their parents reside in my 90g tank, I have had the fry for a couple of months now. I simply had them in quarantine as it was easier to clean up leftover foods. It may have come in on some plants I recently got.

Water changes used "established" water from my 90g tank, which is planted and has only a few fish in it.

Where would one find the anti-parastic foods? Not that I am sure it would do much good against a pest that kills in 2-3 days... I already soak fish pellets with garlic every other meal (plecos get a few, fish get most of them though).

I used to use green malachite, but ran into an ich strain that was immune to it, and used a coppersafe & salt combination ever since.