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I have 6 ghost catfish in a 20g aquarium with 12 zebra danios. The tank is vacuumed once a week with a 25% water change. Today I noticed that 3 of the ghost catfish are developing white, milky patches on their skin. It's mainly occuring near their tails. Where their skin used to have a glassy appearance is now completely opaque.

I've searched the forum for any posts of ghost catfishes getting diseases, but found none. In the past, I've had cichlids develop Ich, but that doesn't look at all the same as what this is...

I'll try to get some pics up, but does anybody have any idea what it could possibly be and anything that might remedy it?

If we're talking about Kryptopterus here, there's little to worry about. In my experience these fish have this occasionally. It's called mechanical damages overhere and these may be caused by swimming against tank furniture for instance. As you know, these fish are not the bravest, so when you clean the tank they'll be quite stressed and flee "blindly", thus hitting objects they would normally evade.
In other fishes you would see scales missing, but glass cat injuries look like white patches.
Don't use any medication. It'll disappear in due course.

I wish I could be more help, but I have not encountered your exact problem. What I will recommend, however, is a big 50 percent water change. Before adding any medications, I find that a big water change is usually the best first move when things do not look right. A few months ago I read about a convention of Public Aquarium officials that met to discuss, among other things, fish health. Their number one recommendation when things do not look right... an immediate large volume water change.
-Shane