ADVICE NEEDED. Goldfish dying off.

[IPA]

@IPA I suspect (and perhaps the OP can clarify this) that the paint is copper-colored, not actually copper.

Actually according to @LondonPond ”It’s a fibreglass shell with real verde gris (copper paint + oxidiser to give it that lovely patina).”

 

[LondonPond]

Just curious, when the fish died did it recently rain? Maybe some of the copper paint got in the water, not enough to kill all the fish but make the ones with underlying issue sick and the water change lowered the level? If you are really curious they make a copper ppm test kit but they may be expensive and hard to read.
Other thoughts with the shape maybe it got too hot or is it possible it was a predator, not catching them but stressing and chasing them and why they lost scales?

We’ve had some rain, not deluges, but the copper didn’t kill the other (single) fish that was in there for over a year so I don’t think that’s it. It’s not like the copper is in regular contact with the water, but good thinking.

I did think about heat as to touch it certainly wasn’t cold (I even thought about dropping a bag of ice cubes in there during the hot days to bring down the temp a little. That could be a factor. They often stayed at the bottom in the middle where it would be coolest (until the 25% water change which now has them swimming the full perimeter at the surface even in the hottest part of the day).

I have seen cats and foxes in the garden and a magpie bathing (poss predating) so that could be it. Blackbirds often have a bath too and woodpigeon/collared dove drink from it regularly which could stress them I guess.

Only the first casualty had scale loss. The others were perfect (beyond their demise)!

 

[LondonPond]

@IPA I suspect (and perhaps the OP can clarify this) that the paint is copper-colored, not actually copper.

Real copper spray paint with accelerated aging activator to promote the green verdigris colouring. So inside the shell is inert fibreglass but the outside has a thinly sprayed layer of real copper.

Custom Creation Paints

Custom Creation Paints new website! Spray on Rust/Corten Steel, Verdigris, Chrome, Coloured Chrome, Hydrochrome and Diesel Chrome.

customcreationpaints.co.uk

 

[LondonPond]

Just as an update for those that care...
With weekly 20% water change with water (from the tap), stopping the feed (during Summer), regular water testing (showing only slight rise in pH to 8.0 occasionally but 0ppm on ammonia, nitrites and phosphates) and a pebble jammed in the nozzle of the fountain tube (creating a taller uneven spray leading to increase surface agitation) we’ve lost no more fish and they seem happy.
Thanks all for your guidance.

 

[AlyssaFish]

Yay! That's wonderful news!