Hi there.If you go to Water Gardening Direct's website they are excellent for helpful advice,reasonably priced products,and you can even email or phone them ref their products.I use them all the time.......but they are just down the road from me!
It is right what Roy says,Koi are most definitely not at all suitable.I have an 800 gallon pond and a 1200 gallon pond,yet I would still not keep koi.They need a deep pond,expensive filtration,a hell of a lot of room and will destroy plants.They grow very big,and it is a fallacy that they will only grow to the size of the pond!
I have a nice mix of various types of coldwater fish.The secret is to get your pond completely set up with plants and filtration first, then add the fish,a couple at a time over a long period,to allow the filter to mature.I have the Green genie set up on both my ponds,which incorporate a UV light to keep the water clear.The site I suggest will tell you all you need to know.