When you decide they type of pump to use, submersible or external, they will indicate their size. You want to turn your water over at least once an hour I think it is. Mine does it about every half hour. So the size of your pond will dictate the size pump you'll need. length X width X depth X 7.5 will give you a general idea of your gallonage. Then it depends too on if you have a waterfall and how much head your pump gives, or how high it is capable of lifting the water. The higher it has to push the water, the less the amount it moves. My pond is about 1750 gallons and I have a 3000 GPH pump. The top of my waterfall is about 3' above the pond and the width of the steps starts out at about 14" and the next three steps get progressively wider. So you have to think about what kind of waterfall you want too. Do you want a trickle or a crashing falls?
There are three things I would say a person needs above anything else. One is some sort of protection for your pump if for some reason the water level should drop or the water cannot get to the pump. I had a plastic shopping bag blow in the pond and block the entrance to the skimmer. I have a submersible with a built in float that shuts the pump off when the water gets low. Also, when and if your pump pulls from lower in the pond, and for some reason the water is not getting back to the pond, it can pump your pond dry. This has happened more than once on this forum. I have read about it. Also it came real close to happening to me a couple of times. I had put a mesh basket in the Skippy to help filter the water in the spring when it was more green than I could stand. Well the filter media got filled during the night and the water could't go through it as fast as the pump was sending it so it began to overflow. Luckily it happened in the early morning and DH saw it and unplugged the pump. At that point, I didn't have the float shutoff on the pump.
Number two is a skimmer. I happen to have a submersible pump with no bottom drain so it sits in the DIY skimmer, inside the pond liner and pulls crud from the surface and it also has Black Knight brushes that trap larger particles of crud. Then there is a layer of filter media to catch the fines stuff so the skimmer box is also the mechanical filter. People that use an external pump often don't also use a skimmer because they either have to have a more complicated plumbing setup or use two pumps, one external which is plumbed to the bottom drain and a submersible for the skimmer. I suppose a external could be rigged to handle both the skimmer and the bottom drain but then I'd think it should be pretty strong.
The third thing is the GFI or Ground Fault Interupter. Electricity and water is scarey to me. I think that this would be the first thing I would investigate when starting research about a pond. I know of a man who electricuted all his fish. Thank goodness it was the fish and not him. If I were you, I'd talk to a licensed electrician before starting any pond project. It is more important to get this right than anything else because we're talking about your life here.
Sounds like you are doing a good job of research so best wishes on your project. We didn't research much of anything and just tackled things as they came. I had been reading this forum about 6 months and collecting articles, books and magazines for years about water features so had collected a lot of knowledge without really trying before starting our project. I must admit, we were very lucky when we did our pond and it worked just fine. We have had little problem with algae or anything else but I work hard at it.
Ours is a DIY homemade setup and DH is a plumber so has a lot of construction knowledge. That helps. He also did the digging with a mini digger he brought home from work. We have a small farm tractor with a bucket on the front so he dug the sand out and put it in the bucket to transport to the back of our property. Knowing we are in our 60s, we can see that we'll have to sell this house before too long and thinking that the new owners may not want a pond, we wanted it pretty simple to take apart and put the soil back if need be. Also seeing the pond is something I wanted, I have to take care of it so didn't want a complicated system or deep pond. So mine is 10 X 13 X 24"-30" deep. We build steps in also so when I get in the pond it is easy for me. Another reason we went DIY is that we are cheap and I enjoy the part where I have to figure out how to accomplish something without spending much. So check out the DIY section of the forum and you'll get some ideas there too.
There used to be a forum member named Greg Bickal who sold a CD on DIY stuff. He doesn't post here any more. It was only $10 and well worth it. I will go see if it is still available and post a link here.
http://www.geocities.com/bickal2000/drain.htm
