I ditto tiller and others. Good advice from all so far. The only thing I would add about chemicals...is that there is one bottle of stuff you really should have, and that is a water treatment for Chlorine and Chlorimines. Chlorine is not that bad if you add less than 10-20% of your total amount at once. The fish can get Gill Burn if they play in the hose spray so put the hose in a area they can't get to. The chlorine will hurt your bacteria colonies in your bio filter but won't wipe em out. After a day or two contact with the air will dissipate the chlorine (it will go faster if you have an airator or waterfall). Chlorimines though are baaaad. They will never leave your pond without being treated with an additive. You can call your water company and see if they use chlorimines. If they do, then you need to get in the habit of using a neutralizer every water change. If they don't then you can get away without, unless you notice your plants being hurt by chlorine when you change the water. "Bad stuff" on the bottom are organic compounds like leaves, fish poop, dead fish etc. As it decomposes it will create a layer of mud called "muck". It's black and smells bad. If your pond ever has a sewer odor that's a sure sign that your muck has run amok and the fish are in danger. When this happens oxygen levels drop and you start getting brown algae and your water will look like rootbeer. Anaerobic bacteria start to go to work in the oxygen free environment. This is a fish death sentence. You should regularly scoop out the majority of the muck leaving as little as possible on the bottom after it settles back down. "Regularly" depends on your pond. More trees, dust, pollen, weaker filter etc mean you need to clean it up more or less than others. There is no rule about how often to clean. To clean out muck, you just need to scoop it out. You can do that by hand, with a bucket or shop vac. One thing I like is this little attachment that goes on the end of a garden hose. The flow of the hose water creates venturi suction that pushes the muck into a little nylon bag. Keep in mind that with any of these options you will stir up the muck and it will take a day or so to see improvement. If you do it regularly, the stirred up muck isn't a problem for the fish. But if you have a deep layer of it you might want to do what destany said above, and move the fish into a container for a bit (filled with pond water!) as all those bad compounds will be spiked during the clean. If your depth is closer to 3 feet I would still think about either getting rid of the goldfish (donate them to a pet store or other ponder) or the koi. With either 3 koi or 11 goldfish you have about the right fishload. But with both, you have about double what you will need in a year or so when they get big. If you are dead set on keeping all of them, you need to plan on losing some to stress or plan on getting a heavy duty biofilter and/or doing regular muck sweeping.

You'll never be able to keep it clean. Even well-run pools get leaves and muck and have to be vacumed. Leaves float and sink. And with the sand bottom you'll have to put scuba gear on because a vacuum will just suck up your sand and leaves. Skimmers work with suction. meaning you'd have to have a large pump... huge one.. with multiple pipings and multiple skimmers and the water will have to be blasted back in from the pump which could get the sand oscillating a bit and fogging up the water. Not to mention wildlife will move into this. You also should treat this sitting water. Don't forget birds might drop in it, animals might drown in it, and the water will eventually just muck up in various ways with various bacteria. I envision a real pain. Too bad you couldn't tap into the water, let it flow to a concrete basin, and out .That way you can keep it cleaner and controlled. Instead of letting it flow all the way to the valley, I'd tap into it higher up and let it flow into a concrete basin and I'd let the water keep on flowing and when it flows to an overflow area it will take the top surface with it.. leaves and all. .. and rejoin the stream. That way you always have fresh water, it naturally skims itself and you have a pool that can be cleaned based on leaves that sink, etc. BUT.. I'm sure there's more ways to accomplish this... that's my 2 cents.

Based on the info you have given and assuming the check valve is being placed on the suction side of your pump, If you are installing your external pump above the water line, you will need your check valve below the water line before your priming pot. If your check valve is above the water line, over time, air pockets can form in the line which can cause you to lose prime. How much below water level can you place the check valve? Your concern about the water level would be the water level dropping in the pond due to evaporation, etc. is valid if your check valve is not enough below water to compensate for different levels. This is another way air can get into the line. This is why some companies state to place the check valve one foot below water level. Your water level could drop a foot before it causes cavitation to the pump. You need to keep the run of flex hose to the pump horizontal or sloping down to the pump. You do not want to start the run horizontally and then go up and back down. At the highest point an air pocket will form at that highest level. If you can monitor your pond water level and keep the check valve always below that level by refilling the pond when it loses water through evaporation, etc, you should be fine. If you are placing the pump below water level then still keep all lines as horizontal or sloping down towards the pump as possible to prevent air pockets from forming.You would not need a check valve in this situation as the pump would be in a " flooded" situation. I hope this has helped.

I couldn't find a site that has pics and names but there are quite a few pictured in the Gallery on this forum that may help you. There is also a forum called, "Name That Plant" on Gardenweb where people who are more knowledgeable than some of us try to tell us what we have. The are usually right on the money. You may try there. Nancy