Fresh Water Fishing Season

How can i stop my fishes die from deadly fungus in my aquarium ?
Hi all. I have a small fish tank with some mix of goldfish, carps and terta. Right from lastmonth, they die one after the other. I dunno whats happening with them. But when asked to the pet guy, he said that its bcoz of the fungus and he gave me a anti chlorine solution. But it failed too. Then i cleaned and replaced with fresh water. It was ok. But suddenly again the same is happening. FYI, now its cold in my place and can the season do these changes ? Please advise. Thanks 2 u all.
Goldfish need large tanks, and carp need deep large ponds. Their digestive system is different from tropical fish and their waste products are strong and come in bursts overpowering any filter. So they must rely on water volume to dilute their waste products until the filter can process them.
A wet/dry filter with a large sump would help quite a bit. You might also add a second empty sump to increase water volume further. Water filtered by the wet/dry would flow by gravity into the second sump and then be returned by the pump to the tank.
Maine Open Water Fishing Season! (2007)