It was a series of events, rather than a single 'disease'. In short (and with respect), you probably didn't do enough water changes / tank maintenance, your tank then 'crashed' as fish died which caused a massive overload on you biological filtration resulting in an ammonia and then nitrite spike (which killed more fish, compounding the problem) then you finished off the last of your biological (good) filtration by adding 'wonder tonic', which wasn't really the most appropriate treatment at that point. The moral of the story is 'maintain your tank and do regular water changes' and these problems can be avoided. Alternatively you can stock you tank more lightly (have fewer fish) if you can't commit to more maintence than you are already doing. FYI the 'cotton wool' was a fungal infection, which only affects stressed weakened fish.
You should now clean out your filter (just rinse it in old tank water, not tap water) remove all 'stuffed' fish and do several large water changes over the next few days. I'd recommend adding some 'filterstart', 'cycle' or similar bacterial culture. Hope that helps (and isn't too discouraging)