Help, Newbie problems.

Hi folks,
I'm new to the aquarium thing, I've been reading all I could find on
the net before I started my tank. Here is what I have done. I got a
55G tank along with a Penguin bio wheel filter and all the other
things
you set up a tank with. I cycled the tank for 6 weeks with 3 Pink
Gouramis. Once all the chemistry readings came out like they should I
added a few fish and all the sudden my Gouramis got vicious with the
new fish. I gave them to a friend with an established tank and they
are doing fine there. Now in my tank are 2 Blue dwarf gouramis, 2
Angelfish, 2 plecos, 2 bala sharks, 3 green corys and 1 black
knifefish. all was good for about 2 weeks after they were all in, then
I noticed one of the corys not acting well. He died last night without
any visible signs of anything on his body. Now I notice that my black
Angelfish has some white stuff on its body, it is irregular shaped and
looks kind of fuzzy. This has me worried. Any ideas on what I should
do or look for? I do regular water changes and have add a small amount
of salt to the tank. Aside from that I don't know where to go with it.
If it matters the fish all came from Petsmart. I hope you all can help
me and thanks for your time reading this.

Duane Wible
Duane Wible