Goldfish sudden deaths

Question
Hi Robyn
We have had a fish pond in our new back garden for the past 2 years, it measures:3.5 x 2 x 1.2 metres deep,the pond is fed by a smaller circular pond 1 mtr x 0.5 mtr deep,this then cascades into the main pond via a 0.5mtr waterfall, the smaller pond is fed water from the main pond via a large pump to a 3 stage Bio filter and UV light. We have succesfully kept golfish ( Sarasa Comets ) for the past 5 years and have bred many young with these fish.In the pond we have a large amount of Oxygenating plant, lilly's and just recently " Duck weed ", I have tried to keep the levels of this unsightly pest down but it just keeps coming back, we HAD approx 20 fish in the pond from 3" to 8" all Comets, a month ago I purchased 10 small Sarasa's from the local waterlife centre, I placed these in the top smaller pond to climatise them to their new enviroment for 3 months.
On saturday I went to top up the pond and noticed one of the larger fish dead in the pond(floating), as I went to remove it it soon became apparent that there were others, in total 7 fish ranging from 3" - 6" in size, also I found 3 small (1") fry dead, from this years breed.
I did notice that the water level had dropped 3-4" and that the waterfall was not creating many oxygen bubbles, so removed the dead fish and I introduced a further pump to create a fountain to add more bubbles,sunday I purchased a water testing kit for all 4 water tests and all were fine, I also brought an Air pump and added this to the deepest end of the pond, this has added a lot of additional air into the pond.
This morning I found another fish is dead,this fish had appeared to be slow over the past couple of days and near the surface of the pond but I thought that it would have been OK as we had not seen it last night near the surface of the pond.
All the dead fish looked in fine condition, with no tell tale signs of ill health, amazingly all the 10 new fish I added to the smaller pond are fine and still swimming around happily, although this smaller pond does have a good supply of water from the main pond pump via another 0.5mtr waterfall, please can you help???

Kind regards

Paul Russell  

Answer
I'm sorry that you lost your fish.  I've lost a half dozen goldfish myself earlier this month for no apparent reason, maybe low oxygen levels.  Has it been hot there too?  I'm guessing you're in the UK?  Anytime you add new fish without quarantining, that has a risk of introducing something.  It sounds like your fish may have suffered from low oxygen levels as well.  If there's a lot of pond coverage from duckweed or other floating plants, that can reduce the gas exchange rate.  You can net some out.  Goldfish normally eat duckweed (I can't keep it alive in my main pond).  So-called oxygenating plants produce oxygen during the day but use it at night.  If there's a lot of those, and it's been warm, that could further lower the oxygen levels overnight leading to gasping fish at dawn.  Did you test the oxygen levels?  What were the results of what you did test?  I have two waterfall systems and still lost fish because the water got into the 80's degrees F here.  You did the right thing by adding the extra air stone which is what I also did.  I still found victims a few days after that, presumably either I missed them, or there's a delay in the die off.  In other words, the low oxygen levels made them weak, and they couldn't recover.  Smaller goldfish have less of a need for oxygen than larger fish.  I hope it was just an oxygen problem that is now fixed (in both our cases)!  If low oxygen didn't kill your fish, I would wonder if the new fish could have introduced something.  Good luck!

Robyn
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