Perhaps I just couldn't see it clearly, if the edges are dark (black) or red, then it is most likely fin rot. I didn't see any black edges and he already has a bit of red coloration in his fins... My intial thought was that he tore his fins on the plant/filter (he may have gotten stuck on the intake while he was resting...I've seen it happen)...but that can lead to fin rot (any damage to the fins can). The plant doesn't look THAT bad...I think the filter is the culprit...but there's reallly no way to be sure!
The change from Sat-Mon is most likely due to a lack of water changes (again this is from my own personal experience). If he's already stressed (and since his fins are missing, I guarantee that he is), he cannot tolerate anything less than ideal water.
The heater should be ok since you've already got it in there and he's ok... Just continue with the treatment and water changes...that's all you can really do for him at this point. It may take a bit of time for him to act normally. Whenever one of my guys is down, a water change usually rectifies the situation, but if they have fin rot, it can take a week (maybe more?)for them to start feeling better (or for me to notice they feel better).