My fish are floating upside down and dying!?

Can anyone please help! I recently had trouble with my 65L where I brought a few new fish and they ALL died. Everyone of them! Some of them had white tufts, some had redness around their gills and faces, most ended up floating upside down at the top of the tank before dying. They also couldn't swim, wouldn't eat, and some had pop eye and cloudy eye. All of the died and since then I added 2 tiger barbs into my tank, and then the next day I added in another tiger barb to keep them company, a gold gourami, and red tail shark and a cory. One tiger barb I found dead on the bottom on its side yesterday, and another one of the barbs I found floating upside down and I put it in a seperate container and it has since died. My gold gourami has either been hovering near the bottom of the tank and sort of leaning on the gravel (not on its side) or at the top of the tank, the other barb is also near the top of the water. The shark and cory appear absolutely fine! The gourami is still eating, but the shark isn't.. I put that down to the fact that its new to the tank and hasn't got into the habit of food being feed to it at the top. The gourami and barb are both still moving around, just hovering a the top of the tank! I've just done at 1/4 water change and added in a multi cure, and before that I'd added in melafix. Yesterday I added in melafix and pimafix. I've taken the carbon thing out. Water parameters are: ph 7-7.2, ammonia 0, nitrite 0, nitrate 0. Water temp is about 25-26. HELP!