Thanks for the comprehensive post longhair, you certainly seem to know your stuff.
I'll acquire a pond testing kit and post up the results to see if anything has gone wrong. I'll also try and give the fish some more shelter and also try changing the food/feeding less to see if that helps. I'll also take some temperature readings and get a better estimate of the number of fish/get some full measurements of the pond.
About the airstone, I have no problem purchasing one, is there any possibility that it could damage the fish if oxygen is not the issue?
I'll post back soon, thank you so much for the help!
My late father owned a koi pond in his garden that has both Koi and Goldfish in. Unlike my father, my mother and I know very little about fish or fish care but are endeavouring to keep our inherited fish healthy.
A short while ago, one of my father's koi carp died, there appeared to be nothing physically wrong with it although it was one of our largest koi. (I'm not sure if this is related to the following but it may be relevant).
I have always noticed that the goldfish in the pond in particular seem quite 'fat' with large extended bellies, we assumed that they were pregnant and I have never given this much thought.
Today I found one of the fish dead, at the bottom of the pond, it had an insanely large stomach and on observation it seems that a few others may be heading that way also. I have attached some pictures below.
Does anyone has an idea what may be causing this? As I have said, me and my mother are no fish experts and we are desperately trying to avoid killing off my father's fish-stock.
The following points *might* be relevant in diagnosing: - The pond is about 6 or 7 ft deep and besides some water lilies there is not much shelter. (I have attached a photo of the pond below so you can get an idea). It is maybe 15ftx6 (maybe more). - We feed the entire pond large fish pellets that I think are primarily meant for koi, all the fish eat them though. - We have about 10 koi, 2 of which are quite large, and maybe 15 goldfish (although this is an estimate and I can recount). - My mother who feeds them often seems to put quite alot of pellets in and I often find them floating around on top of the water a long time after she feeds the fish. - We have a bead filter that we regularly clean out and backwash.