My silver dollars ain't got no tails!

I have a 75 gallon tank with: six large silver dollars (4 inches),
four large goramis, three big bala sharks (six inches), two clown
loaches, a few cory cats, assorted tetras, two tiger barbs, and a
plecostamus. (sorry about the spelling). I keep the tank at 78
degrees. It has been set up for a year. No problems or deaths in six
months or so. The fish get along great, very peacefull tank. But I
suspect it's a different situation at night.... All six silver dollars
have little or no fins left! Healthy otherwise, but they look sort of
goofy with no tails. Every other fish has perfect fins. Any ideas?
Also, about a dozen of my fish are in the four to six inch range now.
Does this seem like a crowded tank? I do fourty percent water changes
every Sunday with HARD well-water. Oh, I feed them flake food and
shrimp pellets sparingly. Thanks, Dan