I hate to be a downer but 3 gpms is not anywhere near enough to keep a 1/2 acre pond cool enough for trout in the summer-- even if you do aerate it. I run 45 gpms into my 1/10th acre trout pond and it still gets up into the low to mid 60's in the summer with well water that is 51.6.
I use plastic media to aerate my well water. The well water drops through five gallon buckets filled with plastic media that breaks up the water, not only adding oxygen, but blowing off nitrogen and other gases.
Here are a few pictures:
The plastic media I use. Basicially you use a size commensurate with the size of the flow you have.
The five gallon buckets filled with plastic media (hole cut in the bottom and plastic screen inserted to hold the media in).
The top bucket has a plate with holes drilled into it to make sure the water distributes evenly across the media. There is media under the plate.
Notice the iron stain. My well water has about 2 1/2 ppm of iron in it. I have to remove the media once a year and clean off iron deposits with muriatic acid. A real pain!
My twin brother (cost accountant/comptroller with a brown he got out of my pond last fall. He hardly ever fishes and wasn't that impressed believe it or not.
A friend of my mom with a big brookie out of the pond a few years ago:
However don't give up. You can still raise trout for food in a homemade tank with that flow. A good book that explains it is Small Scale Aquaculture by Stephen VanGorder.
