I woke up this morning to find 2 of my silver dollars with black all along the tips of their fins and a couple spots on their sides. I did a bit of research and found something like a black ich that can do it, but couldn't find any photos. Some folk on another forum had been talking about it being part of their mating cycle. By the time I got back to my tank it had faded almost entirely, now it is gone. Severe wtf.

Are they sick, or just mating?

http://badmanstropicalfish.com/profiles/profile23.html

This site talks about the breeding a bit...but I'm still wary...

I have gotten goldfish before that have had black on the fins and tips, which also went away after a few days. It turned out that it was ammonia burn on the fins. I don't know what else it could be, but I hope they do okay! Good luck!

There is a breed of silver dollars that have black on the tips of there fins and black spots, I had 3 myself up till a week ago. there is also some red on there side. Look it up spotted silver dollar, there is actually 3 different types Ive seen the normal ones, spotted and red hook. When I had them I definitely had a male, when he got excited and chased the other two his spots really stood out and his spots also. Silver dollars are great dither fish for larger fish tanks, they can take alot of abuse and when they had enough they actually fight back and have a very hard bite.

Ive seen several fish at LFS with pretty gross water have the black blochy spots.... So I am assuming that is the ammonia burn??? I did purchase a fish like that once, and when I put it in my tank, after one day it was all gone... was kinda a bummer, I liked the look, thought it was a cool marble color.. lol

So right now when you look at your silver dollars there's no spots or faint black on there fins? My old spotted silver dollars were fain until they were aggressive or like you said when mating was going on.