Fraying Tails.. Now Slime!?

I had had a tank setup for 3 years now and never had a single problem. Went to PetSmart and got two angelfish from a system where their pumps were failing and stupidly brought them home and added them to my already established tank. There fins were not looking great in the store and once home, deteriorated and basically fell off and they were acting lethargic. I tried treating with pimafix and they got worse after one day. I did a 25% water change and cleaned the top of my aquarium where there was some algae, then added fresh water and put in melafix to treat the bacteria. Now the water is a bit cloudy and there's this white slimy stuff coming out of my pumps which I do not have carbon in and also sticking to some of my live plants!! I added a little bit of aquarium salt in case that would help. I don't want to lose any of my just purchased fish or my older fish which now have some kind of deterioration on their fins as well. The fish themselves are swimming around and acting happy, but this just cannot be good! Even a fish I never have any issues with has a bit of deteriorating on the tips of her fins... Please help! !!!!!!!!!

I wonder if its fin nipping?

Melafix and Pimafix are both only useful as a preventative. As they're based on tea tree oil, they can cause some slime-type deposits as the filter emulsifies the oil and water.
 
I would do a very large water change (at least 75%); clean water is the best medicine for fish.
 
To help us get to the root of the problem can you tell us; the size of the tank, exactly what fish you have (numbers and species), what filtration you have, how often and how large your water changes are, and any test results you might have (pH, hardness, ammonia, nitrite, nitrate).

These are new fish that I purchased. So it is not fin nipping.. it is from poor water quality at Petsmart that is spreading throughout my tank. I did not have any water issues before bringing these two home and now 2 that had no problems before have something going on with their fins. I have used pimafix and melafix successfully in the past to treat issues that I may have had.
 
Funny enough, the two that were lethargic and sick look are looking better and swimming all over the place and my previous fish (koi angel) is the one looking sick now. He is floating around at the top with ragged fins that are looking weird like fungusy or bacteria. I do this every time and I always say I learned my lesson, I will start a hospital tank and quarantine before introducing new fish and then I cant seem to get a hospital tank going. frustrating.
 
42 gallon, heavily planted (VERY heavily planted and with drift wood)
6 angels (1.2" only), 4 gold tetras, 2 cories, 2 kuhlis, 1 bristlenosed pleco
over the tank filtration x2
25% water changes every week or so
6.3 ph
0 ammonia
0 nitrate
0 nitrite
40 alkalinity
200 hard water =(
 
SO are you saying to water change and stop treating? I'm so nervous to do that..

oh the weird slimy stuff has gone away. so it's just back to fish issues.