Probably because they saw something that frightened them, like a hungry
bird or other predator. I noticed below someone suggested that your
pond water need to be 1 year old before introducing fish. Well, that
may be a bit long to wait. When I first started up my pond (2 seasons
ago now) I waited 7 days before introducing gold fish from my next door
neighbors pond. He was wanting to get rid of some anyway and I was in
need of some. Anyway, the fish were fine, did not hide or show any
signs of distress. After that, I began to gradually introduce a couple
of koi at a time (once every couple of weeks or so). No problems. Just
give them a little time. They will come around.
Happy Ponding!
W. Dale

googlefan wrote:

Hi,
I'm a new pond owner.
About 2 weeks ago I bought 6 goldfishes and the first day they seemed
to be fine, but after that they always been in hiding. 3 days ago I
bought 5 more goldfishes and pretty much the same thing happened, at
first they were very active but the next day they went to hiding with
the "old" guys.

It's a 160 Gallon pond with a lot's of rocks. I'm using underwater
pump and filter kit, which includes Biofilter. I tested the water at
the local Petco and it seemed to be fine.

Any ideas why fish will be hiding all the time?

Thanks in advance

Googlefan