Interesting pond.

Is the area under the deck a biological filter? The waterfall should run 24/7. Do you have a water testing kit? Did the fish have any injuries or appearance of illness? The fish on top of the water indicate poor water quality including lack of oxygen. Run the waterfall. What kind of filter system is there? What is the water temperature? Is the bottom clean? Does the filter need cleaned? Turning off the waterfall if the filter is behind it will cause beneficial microbes to die within a few hours.

That is a lot of fish even for a pond that size. If you turned off the waterfall the fish might not have had enough oxygen and it would have messed up the rest of the water quality too. Testing the water tells you pretty much what is wrong. A decent kit runs about $30.00 or so.

The fact that the surrounding lawn drains into the pond is a problem. Anything put on the lawn drains into the pond. Chemicals good for the grass can be deadly to the fish. Tap water can be really bad too. If it has chlorine or other water purifiers in it, it can harm the fish. If your tap water is safe it should still be added slowly and sprayed rather than run in. This allows some chemicals to be removed. It is better to add it from a holding tank that has been allowed to sit for several hours. Even better is if it is churned.

Don't add anything until you know what the problem is. For now, run the water fall and test the water.If you have frequent rain the runoff will mess with the water quality as it drains from the lawn. Until you know what is going on you should test after every rain. Do you know where excess water goes?

You can do a search for specific information. The box is at the top right of the forum page.I am adding a somewhat related response to a question that may be helpful.

" RE: Koi jumping out of ponds

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Posted by sleeplessinftwayne z4-5 IND (My Page) on Wed, Nov 7, 12 at 6:08
Fish produce ammonia. Ammonia is toxic to fish. It is also uncomfortable so that might cause them to jump. Fish poop, excess food and organic waste rotting on the bottom produce nitrites and toxic gases as well as being a home and nutrient source for anaerobic bacteria. That may not cause them to jump but it is unhealthy for the fish and for humans. (You might like the link below.) Ph is affected by many things including rain, organic matter etc. Koi do not tolerate water that is too acidic. They do much better in alkaline water. Nitrites and nitrates in the water will cause water problems and feed algae. KH is a measure of buffering capacity or resistance to change in PH. Knowing the levels of these is important to avoid a crash that can and will kill fish.That is where a test kit comes in.
There are 3 types. The first is a research grade kit and while extremely accurate it is expensive and you don't need that level of results. The second type are strips that come in packs of fifty, are easy to use and can be frustratingly inaccurate unless fresh, properly stored and kept absolutely dry until you use them. Most of the ponders on this forum don't use them although a few do. I tried them, didn't like them. Half of what I bought were not accurate. The third while not as accurate as the first is totally adequate for your purposes. It contains liquid chemicals, test tubes color charts and instructions for the tests you need to do regularly. They run around $25 to $30. It is recommended that they be replaced each year but I have cheated and had good results when the kit was carefully stored. I don't push it though. It is not worth it to me if I lose a koi because of it.

The next part is from a web site but is correct.
"When to test.
An established pond with healthy Koi fish should be checked every month. It is only when you notice something out of the ordinary, and possibly during seasonal changes, that an additional test or two might be prudent.

An ounce of prevention is worth a pound of cure. A simple test, at the right time, can prevent a minor problem from becoming a catastrophe.

When starting up a new pond system, daily tests may be required then weekly for a couple of months until the system has stabilized."

Right now your main concern should be ammonia. You have a lot of fish. Limits on the number of fish is usually figured by gallons of water to inches of fish. Since some of your fish are rather large you may very well be approaching the edge of disaster. The test kit will tell you. To reduce ammonia, you can aerate with a bubbler, agitate the water as with a waterfall or fountain or use a product to remove it chemically (I use a combination of Ammoblock and Activated carbon in a bag in the Skippy filter. It does a good job for my little pond) In any case, you need to test the water.""

Excess nutrients in the water will feed algae. Any fertilizer put on the lawn will wind up feeding algae. That may be one reason why the water is not clear. Since you have been turning off the waterfall that is complicating the problems. If running the waterfall and filter does not start to clear the water in a couple of weeks then we should revisit the question.

Your pond is beautiful. They are work but very rewarding. Can you contact the previous owners-find out if it is a liner pond, water source, types of fish and plants etc. If not, someone in your area may have a pond and would be happy to come by and give you some help. Ask your county extension person, garden groups, nursery personnel etc for someone they know that has a pond in your town.

I know that I would be happy to share some of my limited knowledge with someone new.

Love your pond but the above comments are valid. Lawn chemicals will create an issue for you. If you must turn off the falls, you need to add air somehow. A pond sized bubbler may help aerate your pond. Oxygenating plants will be a must for you as well. Adding a berm with plants such as tall grasses and rocks may solve some of the issues. Also check for waterfall leaks. You shouldn't lose that kind of water by running the falls.

Back with a few more comments and questions. I'm afraid I might be overwhelming you.

I would be more worried if the water loss occurred while the waterfall was off. Without seeing the way the falls are constructed it is difficult to figure out what exactly is happening. When the waterfall is running, water is in the pipes and tank. When the falls are off, that water drains back into the pond and raises the level. Depending on how much water is in the falls system you may not have lost any water at all.That tank behind the falls looks to be pretty large. From what I think I see it seems there may be a bottom drain that feeds water back into the falls system. Depending on location and the size of the pipe, that could be a lot of water. To determine if there really is a problem allow the waterfall to run until you no longer see the water in the pond getting lower. Then top it off. Don't turn the waterfall off. The water level should remain stable. If it drops unreasonably, then you can be concerned about a leak. If you turn the waterfall off when the pond is full the pond will probably overflow. Because the pond is below grade you may not notice it.

Can you describe the waterfall system? Are there any controls to change the amount of water going over the falls? I'm trying to understand why you want to turn them off.If it is too noisy, you might be able to divert some of the flow so it is quieter. Most people work to increase the noise but I can see the shape of the hill might amplify it.

The bubbler is a really good idea but your pond was designed to work with the waterfall.

Feed fish only when the water temperature is 55oF or above. They may not want to eat if the water quality is bad. They may eat at lower temps but it is not good for them. They often graze on algae that grows on the sides and bottom of the pond. That is OK. If you can't feed them for a couple of days or even longer, they should survive just fine.

Check the library for a book on pond fish to identify what you have. It sounds like you have Rosies and Comets but I don't recognize the red ones with black spots unless they are young Koi. Depending on the country where you live, you could have something totally different.

Where do you live? What is your current weather like?