Alright, I hate to be the bearer of bad news but your tank is severely over-stocked
. I'm guessing you were given incorrect information at some point, maybe at the pet store?
For fancy goldfish it's generally recommended to have a minimum
of 20 gallons for the first goldfish and then an additional 10 gallons for each additional goldfish after that.
What's happening is that there's so much ammonia being produced by your livestock that the nitrifying bacteria (the guys who convert ammonia into nitrite and then nitrite into less toxic nitrate) can't keep up. The tank is also so small that the toxic ammonia isn't being diluted much at all. Your large water changes are probably the reason your fish are alive at all but they're still being burned by the ammonia. The burns are then being infected, leading to the fin rot and streaks.
No amount of medication will fix this problem and the tank will eventually "crash" if nothing is done. Again, I hate to be the one to tell you this
The only way to save your fish is to upgrade to a much, much larger aquarium or bring them back to the store (and get smaller fish species that are more appropriate for your 11-gallon).