Hi Albie
I have lots of experience from my leaky pond (Not something i like to brag about)
Seeing as you have quite a "big" leak i think you can run tests in 6 hour checks. 6 Hous sound long but these tests can shortcut the time it takes to find a leak. The alternative is to leave it and wait, wait....until it stops.
Please post a photo of your pond and describe its workings like number of suctions lines, return lines, filters ect...
First thing - use a log book because you will get confused and wont remember what the readings was when you did this or that. These test will propably have to happen over a couple of days unless you are awake for the next 6 hours from each test.
You have to determin if the leak is helped on by the pump or not.1. Fill up the pond upto its normal level and mark it.
2. Switch off the pump off for 6 hours. After 6 hours check the level again. If it dropped measure with how much and write it down.
3. Refill the pond upto the full mark. Now switch the pump on for 6 hours. If it drops mark the level and measure.
4. Compare the two. If you had no leaks at (2) then its on your pump line somewhere. If you had more water loss on (3) than on (2) then you have a leak on both the pond and the pump lines.
To determin if your water feature is leaking1. Fill pond to full mark and take level.
2. Let it run for 6 hours with only the waterfall on.
3. Take level.
4. If it dropped refill the pond to full mark. Bypass the water feature by extending the pipe that drops the water on the water feature and take it directly to the pond. I find that the swimming pool type kreapy crawly pipe works very well for this.
5. Now with the waterfall bypassed let it run for 6 hours and then check the level.
6. if it still leaked the same amount then its not the feature but might be the pipe feeding the waterfall that leaking.
Now to determin if your bottom drain line is leaking..1 . Buy a 110ml plug from a hardware store its a device that looks like a trolley wheel but the wheel is made of rubber. On the back of the wheel is a screw as you tighten it the rubber wheel gets expenanded. You need to dive down put that in the 110ml pipe of the bottom drain - tighten it enough so no water can get pass it.
2. If you have a settement chamber empty it and check that no water comes in via the bottom drain line.
3. Wait 6 hours and check level - if it drops then its not your bottom drain . (Depending on how well you thightened the plug)
To determin if one of your underwater return lines are leakingThis test can also be used for testing most devices installed in the walls of a pond likes skimmers, lights, mid suction lines, under water return lines.
1. Fill the pond to about 2 cm above the water return and mark it.
2. Switch off the pump and wait 6 hours.
3. Mark and measure.
4. If the water level dropped then remove water out of the pond until the water no longer touches the return line. It must be just-just below it.
5. Leave for 6 hours.
6. If the level dropped then your leak is lower down. If it stopped leaking then you propably have a leak on that return.
To determin if your gravity fed settlement chamber or vortex is leaking.1. Switch the pump off.
2. Using a stand pipe or some device block the incoming lines off.
3. Block the suction lines off.
4. Mark the level and Leave for 6 hours.
5. Check the level.
Remember you can have leaks on several of these places and they can all be addin to the your big leak.
Also remember that if the leak is lower down on your pond it will slow down (leak less) the closer you get to it as there wont be as much pressure on it. So this can cause you to think that there is a leak on a device even though there is not.
I hopes these tests help for now. Keep us posted.