Jimmy as I mentioned earlier in order for a treatment/medicine to be successful the nitrites in the water have to be eliminated. I truly understand your frustration and desire to quickly deal with the disease(s) but with either ammonia and/or nitrite present in the water no medicine will work. It's that simple.
It's important that you comprehend the ramifications of tank water that contains nitrites.
Listed below is a link, click on it, which will supply you with some fundamental information re: nitrites.
Another possible drawback to adding medicines whilst there is nitrite in the water is that some, not all, medicines will kill the beneficial bacteria needed to consume nitrites!
You wrote-"The bacteria is obviously in my tank to stay.....this I think will wipe it all out for good???". If you're referring to nitrites this is not true. With the appropriate measures taken and some patience nitrites can be eliminated but if the proper stimulus is present in the water then nitrites will indeed form and wreck havoc as you have witnessed.
You also wrote-"Maybe I might be overfeeding. How much food should I use for this many fish?" Most likely you were/are overfeeding and overfeeding is definitely a stimulus for nitrite formation. For now feed the fish only a small pinch of food once every other day. When the water parameters return to normal you can then feed the fish a small pinch of food once a day.
Now as an option what you can do is seperate a few/one of the sick fish into a smaller quarantine tank using all new dechlorinated water. This water will obviously be ammonia/nitrite free thus providing the foundation for proper treatment.
It is in this tank that you can administer the medicine that corresponds to the particular illness you are treating for but remember that in order to not let ammonia/nitrite form you will need to do 50% partial water changes every other day in the quarantine tank.
Keep in mind that you must keep a fish(es) in the main tank because if all the fish are removed from it the whole biological balance will collapse and you'll need to start all over
again and if you think 0.1ppm nitrite is problematic you haven't seen nitrite levels in a newly set-up tank (tank with all new water)--.4ppm-.6ppm!
Ok-- as for the medicine JBL Furanol, it's ideal for use in treating a wide variety of bacterial aiments.