I remember of a similar thread some months ago but I cannot find it now.
I think that it's called something like Hole in the Head disease. If I remember well the cause is not known but it's suspected that it may be caused by prolonged use of activate carbon.
That is a slime coat issue. It is caused by water conditions. My guess as you are a long time fish keeper there is a build up of waste in and under the rocks in your tank. They should be rinsed occasionaly The waste accumulates in that style gravel and your fish keep growing so there is more all the time. This is the huge advantage of sand.No small pockets between the particles to collect this. My guess is only based on the pics....Or if thats clean hard water or chloramine type burn from a drastic water change but the first guess is based on the apparent age of your tank....
Do not clean all at once to avoid a crash.Set rocks in a bucket with some water to avoid fish flopping out of holes to a dry landing after being removed from the tank.
Tinman, You may very well be right. I failed to mention I had to drain 80% of the water to move it (new carpet and paint had to go in). Our water is VERY heavy in Chloramine. I put a little extra reducer in when refilling. Not enough aparently.... What can I do to treat this? I feel absolutely terrible about it....
Is that lava rock in your tank? It looks quite rough. I would seriously think about changing that over to a smoother type of decor for the loaches sake. You might want to change over to a sand substrate at some point too, because not only is it gentler on the loaches barbels, but any waste will sit on top and get taken up by the filter/s. With gravel, a lot of waste can go unseen, where it builds up and causes problems.
A side question. What is the PH? The fronts and loaches are at opposite ends. I'm assuming it's low and the fronts have adjusted? I keep both, in separate tanks.
"Beware of the fish people, they are the true enemy."
-- Frank Zappa, speech to a pro-choice rally in Los Angeles around 1989-90