Hi we have a middle size garden pond holds about 2000 litres. This last month we have found that the fish are staying at the top nearly all the time. We have approx. 25 fish ranging from 4inches to 10inches. They are all some type of goldfish although lots of different colours. We have a fountain in it and also two water pumps to keep the air flowing. One of the pumps works a waterfall, this has not been working this year. Could anyone help please?
Wingsyorkshire

I believe the cause is not enough oxygen in the water - do you have plants and a filtration system?

Definitely due to not enough oxygen in the water. You might want to invest in some air stones (you'll have to buy for a large tank, these aren't available for ponds.. purchase for 125-150 gal tanks) if you don't plan on starting up your waterfall.

Water circulation isn't enough to put oxygen into the water. The waterfall is a great example of putting oxygen into the water.. Air is pushed into the water by the waterfall.

You also might want to get your water tested. Goldfish/koi are dirty fish. They produce a lot of waste and will actually create toxic water. The poisonous stuff sits at the bottom creating an unhealthy habitat for the fish. If it gets bad enough, they will start to go to the surface for air.. If it gets really bad, they'll actually jump out of the water to try and escape the filth. There are products out to help regulate ammonia levels in ponds. You can find them at any garden supply store that carries pond equipment..

Hope I helped!