The fish are carp/koi cross. the shubunkins are 6 inch long
So the new fish and original fish are/were basically the same size, 5-6". Good info.
my pond is triangular in shape. each side roughly 2.7 metres. the pond graduates to a depth of 1 m. I calculate my pond is between 3000 and 3500 litres
As a very rough estimate 3000 litres could support about 100" of fish. You had 34" worth of fish, so over crowding can be crossed off the list. However, it is still possible to have an ammonia or nitrite problem so it's still worth measuring from time to time just to make sure.
I have a lilly, and oxygenator and some iris versicolor
Should all help to handle ammonia and nitrite.
when i put the fish in the pool they went very quiet/still. One went to the bottom and was hardly seen (that one died yesterday) the other was continually chase by the two largest shubunkens. Incidentally these fish seem to dominate the other two.
This is kind of important and I'm reading two different things. Both fish went quiet but one was chased. Not possible.
This point is important because it's really the only good data you have. Going still and going to the bottom would point to:
1. Fast pH change the fish couldn't handle.
2. The bag they were in was out of O2, or too much CO2 and fish was so weak they couldn't really breath and sank to the bottom where O2 may have been very low making things worst. Fish have to work much harder than we do to take in O2. Death can be slow, just like in humans who have a hard time getting enough O2. And a build up of CO2 in the bag can drive down pH and burn off the slime coat.
I don't know if any of this happened or not. Without measurements it's impossible to tell. But it does just about rule out ammonia and nitrite as the primary problem because it happened so fast. Plus no other thing points to an ammonia problem. Ammonia test is still needed. Also probably rules out parasites, bacteria and virus. It's still possible the fish were in some compromised state, weakened by the trip home, and succumed. But unlikely given your description of the fish in the shop tank.
The other important thing is when did the chasing start? If it happens in the first 5 minutes, and since it happened to one fish, I'd have to keep thinking spawning behavior. But the Ghost Carp in theory shouldn't be putting out spawning pheromones at 5". There could maybe just be a mix up and the your fish smelled something that triggered spawning behavior. But I would think that wouldn't last long, that they would figure it out. There is also the possibility these fish are stunted and older than their 5" length would show. Some people say carp can spawn at less than a year old and even at 3 months, but I've never found real good sources. It does depended on water temp. I'd keep spawning as a possibility.
Have you seen spawning behavior before? Meaning do you know what it looks like? When most people see it for the first time they describe it as an attack.
I've never heard of goldfish attacking other fish to the point of the fish dying from injuries outside of spawning. Goldfish might chase for a second or two, but rarely even make contact. Goldfish aren't territorial, fin biters, etc.
I dont think my pond is heavily stocked. I can see the highly fed active pool in the shop would have more ammonia than my pond.
I also don't think you have a large fish load.
As a side note, the shop should have proper bio filtering to handle the ammonia load or they risk losing all their fish within a day or so. This is why pond size to number of fish isn't a very good gauge at all. It's more about how much food is fed and bio filtering capacity. Larger ponds do dilute ammonia more, but that's a very short term thing. If ammonia is building in a pond it will be slower to build, but it'll get there. And a larger pond has more algae and surface area so more opportunity to consume ammonia. Larger is better, but not the only factor or even a super important factor.
