Help with my silver dollars. What are they doing?? pic

[Lynsey]

Ok, I got 5 silver dollars total from three different stores (since it is very hard to find large ones in my area). I have had them for about a month now. Three of them have developed dark spots on their dorsal fin, and all of them have developed more red on their fins. I thought this was due to the better water conditions and feedings that they don't get in the LFS. But two of them have developed a darker spot on its side and the end of the tail and dorsal fin is now very black (which it did not have when I got it). These two are are also stuck to each other like glue. They often go in the corner of the tank and vibrate against each other. They also chase the other silver dollars around a bit. I think that these two came from different stores, but I am not sure. I just wanted to make sure they are silver dollars (I have never seen silver dollars with black on their fins). And what does this behavior mean? Just want to make sure everything is normal with them.

Here is a pic of one with the black:
http://image1ex.villagephotos.com/2140264.jpg

In this pic there is one with black and one that does not have black.
P.S. The ones with the black are not the biggest ones in the tank.
http://image1ex.villagephotos.com/2140265.jpg

[pinballqueen]

If I had to take a guess, I'd say they were spawning. Congrats! I could be wrong, though. Any other takers?

[VoodooChild]

I'd shoot for spawning too. The only time I've seen shaking mean anything other than courting is when Africans do it over territory. I doubt though that that is the problem here.

[Faramir]

Could be a dominance display. My male bleeding hearts do this.