I have a large pond with a lot of fish that I've had for about 10 years I have a mixture of gold fish tench and large Koi I havent seen them since about November as they go down the bottom of the pond over winter but I have started to see a few this past couple of weeks and have just started to feed them a little bit now the weather is slightly warmer but my problem is I have had 5 of my big gold fish die in the last week they look healthy by that I mean they dont look deceased or have that fungus on them and they dont seem ill before hand one day they all look fine then the next day they are dead I was wondering if anyone here keeps pond fish and if they are having any problems ,I expect to loose a few now and again with old age etc but this seems strange.I have a small fountain and the pump and filters are working to keep the oxegen levels up so I'm at a loss about why its happening.