Yeah you can use them. Are you sure it's fin rot? I just ask because a lot of people don't know about fin biting which in more cases than not, it ends up being fin biting. Here's a thread to help you determine; http://www.bettafish.com/showthread.php?t=346377
And you can just cut the leaf into strips and use the strips, usually one leaf is a little too big for most tanks lol Usually a strip is good for about 2-4 weeks depending on how fresh the leaf is and all that jazz. I just take them out when they start to look sort of nasty. I usually keep some in though for something like a shrimp tank, they actually eat all the lovely bugs on the leaf (don't worry, the insects won't harm your fish!! He'll eat them too if he finds them
) so it won't hurt if you leave it in longer if you want, you can take it out when you feel like it.