Hello, I am Dr. B, a licensed veterinarian and I would like to help you today.
I am sorry to hear about the goldfish losses you have experienced with this pond maintenance. The situation you have described is something we see with fish who are exposed to significant water temperature changes, are over-stressed and those who are oxygen deprivation. We can also see similar signs with nitrite or nitrate poisoning; but if the pond has just been cleaned out this would be less likely. As well, I assume that the pond cleaner didn’t use any cleanser that may have left a toxic residue here therefore unless you are aware that they did use anything, we can put that lower on our differential list here.
Now stress in fish can arise from any sudden change in their environment. If the water was completely new, then this can be the source of the problem. We can see problems with total water changes due to temperature fluctuations (ie "cold shock"), pH shock, and water hardness alternations. Goldfish are quite robust but we often find the larger goldfish do appear to be particularly sensitive to rapid temperature, oxygen, and pH changes. (Which is why I am not surprised to hear that the largest ones are the ones who have died). We can only speculate why this is, but it may be because they have the largest oxygen requirement of the fish in the pond.
As well, we can even have situations where the previous pond water was terribly toxic (with high nitrates/nitrites/ammonia); yet when we change it to new water we end up with fish that cannot cope with the dramatic change. This is why in nitrogenous toxicity situations, we have to do gradual water changes to wean them down to normal, clean water without inducing fatal shock to their systems.
As well, we need to consider the oxygenation ability of these fish. While fresh water should have a good oxygen supply, it is possible that the stress of change has led to the fish having an elevation in respiration and this then lowers the oxygen resources of the water. As well, if the stocking density in this pond was at maximum for the pond’s oxygen carrying capacity (ie ideal water volume is 30 gallons per inch of goldfish), we could see this problem with the fish having to share the smaller holding tank which would have had a lower volume of water present and thus lower volume of oxygen. As well, if the oxygen balance in the pond was reliant on the filters or any supplementary aeration, these being off could also play a role here.
In this situation, it would be ideal to check water temperature and water parameters (ammonia, nitrites, nitrates, pH, hardness) and it’d be even better if we had pre-change levels to compare it to (to see if there was active nitrogenous toxicity present that has led to a system shock). If anything is amiss, then it needs addressing. Since the ideal water temperature for goldfish is between 68 to 75 degrees, you do want to make sure the water is within this range. If it is not and you need to raise/lower it, do consider keeping to the lower end of ideal for them since they will have a lower body oxygen demand in the cooler temperatures (which may just give you a chance to get the oxygen angle sorted). Otherwise, you do want to take steps to make sure there is adequate oxygen in the water for these fish. This means turning the filters back on (if they aren't already) and potentially adding further oxygen supplementation (ie add airstones, etc).
Overall, it sounds like the larger fish died due to shock and potentially under the stress of temperature shock and/or compromised oxygen availability. Therefore, while I suspect that you are likely out of the woods now that the other fish have settled, I would still suggest checking water parameters +/- oxygen levels (if you have the equipment) to make sure there are not residual issues. Furthermore, I appreciate that the landscaper has done a full change on your pond like this before, but I have to say that I am surprised that he hasn't had mortalities before (perhaps the weather was warmer last time, the hose water closer to ambient, the fish smaller, etc). Still I would strongly suggest that he alter his protocol in the future now that this has happened. Ideally, we'd want him to just doing a partial change (perhaps more frequently), potentially treat the water (depending on its source), and let the new water get to ambient temperature before adding to the pond to decrease the risk of recurrence of this tragic event.
I hope this information is helpful.
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