Please Help My Fish Are Dying

[DCB]

I bought a peacock eel and 2 dwarf gourami last week and on wednesday my eel started swelling up and swimming on his back and side, unfortunatly he died wednesday night , at first glance the rest of my fish seemed ok but now my male dwarf gourami is sitting on the bottom of the tank and leaning slightly to one side, hes swimming normaly and feeding but im still worried about him.
Last night i did a 50% water change using waterlife haloex dechlorinator, i also treated it with interpet anti internal bacteria,

just done a test-

NO2/ 0
NO3/ 20
GH/ 8d
KH/ 3d
pH/ 7.0

when i bought the eel i noticed there was another eel in the tank with him that looked at bit lazy, iv bought fish from there before and never had a problem so i don't know if this problem came from my LFS or if im doing something wrong.

* i do water change 25% every 2 weeks
* I feed flake food 1 pinch daily, frozen food once or twice a week
* I cycled for nearly 2 month before adding fish
* I introduce fish 2/3 at a time and wait at least 2 weeks between adding



its a 3 foot long 100L

fish( in order put in tank )

4 zebra danios
1 common pleco > (moved from my other aquarium)
_______________________

1 pearl gourami > (bought from LFS)
2 juvenile angel fish
2 dwarf gourami
also 1 peacock eel that I bought 1 week ago that died yesterday

please help asap i dont want to lose any more fish.
should i do another water change?
iv got a floating breading tank shold i put my dwarf in this so he doesn't get beaten up by the other fish?

 

[glolite]

Can you do an ammonia test? When you say you cycled for two months, which method did you use ie: fish in or fishless cycle?

 

[WILDER]

Does the fsih look pale or darker in colour.
Can the fish maintain balance. Does it turn upside down when it trys to swim.

 

[DCB]

i cycled using fish in method(zebra danios) i havent got ammonia tester just orderd 1 of ebay

he seems to be a lot better today hes swimming and eating normaly, hes still sitting on the bottom of tank but not as much as he was and he isnt leaning to 1 side, his colour hasnt changed

hopefully hes pulling through

 

[WILDER]

It sounds like it could be a bacterial infection.