Thanks for response, I will watch it closely but I think it is down to decaying vegetation.I will mention that when my small container pond got yucky (I don't know what happened) it developed an oil slick looking film on the surface -- iridescent, rainbow-y looking. No water changes would fix it. There were no mechanical parts in it whatsoever. I think it was something to do with bacteria and decaying matter messing up the water.
I have a few goldfish and some Rudd that seem well enough. The large black Tench was found on the lawn about 5 foot from the pond and was barely alive, just slight movement with the gills and body curved. Now dead.No decay on fish or marks where a predator would have had a go at it. I think the pond is about 500 galls.If this is a sudden problem, I would look at what happened to your Tench. Did you find a dead fish and an oily surface? The fish could have decayed and well... You mentioned smallish pond. Don't those fish grow to nearly 3 feet? How many gallons is your pond and do you have any other fish?
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