[Windmills]

Hello, I'm hoping very much that someone can help me with my problem. I very much love my angelfish (Sachael is his name) and I hope that you can help me to help him!

Anyway, here's my problem:

I have one angelfish in an 80 gallon tank. (see my signature for tankmates) He is about 6 months old and about silver-dollar sized. He has been fine up until about a month ago, when for some strange reason he started wedging himself against a plant and the glass and staying there, as if sleeping, for 20 or 30 minutes or more. Then he would go around and swim a bit, and then wedge himself in the same spot again, over and over.
Around this same time, his tail and fins started looking frayed, with some white stuf on the edges of the fins.
He still eats just fine, though.

What is going on and how can I help him? I don't have another tank to isolate him in, and I'm afraid of adding medication that may affect my other fish (see my signature for other fish) All my other fish look healthy and fine.

The tank itself is over 1 year old. pH 6.8, Ammonia 0, nitrite 0, nitrate hovers between 20 ppm and 40 ppm. I do weekly 20-30% water changes.

PLEASE, PLEASE help Sachael!