Well it's known that in the wild, fish do not get a well balanced meal everyday. Some people (me included) believe that it's good for their digestive tracts and helps maintain a healthy weight.
Also, some medications damage the beneficial bacteria in an aquarium, so during treatment it's reccomended that you feed less.
I just saw that it was a BN pleco, so you're right.
More than likely, it's just plain old stress-
The solution would be to add more fish, but thats not an option due to your tank size.
You could increase oxygenation with an airstone. Might help a little, likely won't cure anything.
Also, I'm not encouraging you to use salt or the ich clear, because from the little information I have, a parasite is the least likely. It's definately not ich. I don't think ich has a glittery look by any stretch of the imagination. The spots could have just been slime, caused by stress or injuries.. Also, 3tsp of salt isn't really enough to do anything in a 20G tank. Normally it's 2-3 tsp PER GALLON, but again, I wouldn't do it.
Right now you have an anti-fungal and an anti-parasite (ichaway). It may not be more than a shot in the dark, but I think everyone's consensus is that it's possibly bacterial.
The shredded fins (whether it's fin rot or not) mean there is fighting. Fin rot is caused by stress, but without an injury, the fins won't get infected to start with.
I definately agree you need to hook your filter back up if you haven't already. The only time you should remove the filter is if the charcoal can't be removed, even then, you should test ammonia/nitrite daily and set up an airstone.
The ammonia spike alone could have killed your recent fish. The triple sulfa would probably work well against bacteria IF that's the problem. Even so, you should really test the LFS. Do some research, then ask them questions that you know answers to. I've found that the nicest people are usually not the best informed. (There are exceptions). Many of the employees aren't trying to make a sale as some people believe- I think most are genuinely trying to help. But they're basing their help on what the bottles, packages, and medications say.
Something like general cure is generally shunned by the community- we consider it a jack of all trades but master of none.
On the other hand, if you have no idea what disease your fish have, and time is of the essence, it might be your best bet. (compared to dumping ich-away, pimafix, salt, and triple sulfa in all at the same time. I know you wouldn't do that, but just the point.)