Welcome to the forum! I had a similar issue recently. 1 discus out of 6 started showing a little bit of fin rot just like yours. All I did was to make sure I did 50% daily water changes every day, I cleaned the filters out real good and I added 6 tablespoons of salt per 10 gallons. It showed improvement within just a couple of days. I'm not sure what's going on with the others, but it could be related. I doubt this is related to the heater malfunction, although it is possible if there were a bacterial issue that bloomed with the higher heat.
You have a planted tank, so I would clean everything real good... the filter, tank sides, gravel, etc... I would not introduce another antibiotic for a coupe of weeks.
And, take a look at this post and it may hep to make sense. Nearly all discus issues are water related...
http://forum.simplydiscus.com/showth...-Are-Important