Debris won't get sucked into the pump?
1) Check if it's algae and fish poo. Sometimes fish poo gets tangle and embedded in algae, therefore wit's harder to get sucked in. You might want to scrub to remove some algae. I think you get a lot of sun in your area? This along with your nutrient rich water (excess dissolve food and fish waste) is ideal for algae.
That's why you need to do regular water changes. Some out, some in.
2) You don't have good water movement. Try a leaf on the waterfall, and see where the current takes it. That's just for surface. For below water, try a bio-ball, and see where it gets pushed. This will give you a good indicator on the water pattern and if you have a dead spot and how fast/slow the water is moving.
3) Move the pump apart so it collects at different areas of the pond instead of at one spot. I think one of your pipes(green) is fixed in place? See if you can spread the green one away from the blue one.
4) If your pump is rated at 8000lph, and its not sucking in a lot of water, then it might A) have debris stuck inside which is impeding flow. Dissasemble, clean out and reassemble. Odds are, when, you turn off the pump, you might get some back flush. B) your getting too much pipe/hose resistance. The hose is too small or/and too long. As water moves along the hose, it will get more and more resistance, slowing down significantly, especially if there's head height involved. So you will need to shorten down the pipe, or make the pipe bigger. What size hose/pipe are you running?


**** After thinking about this, if you have the pump cleaned and assembled, and it still doesn't have a stronger pull of debris around it, then I think it's only pulling water from above, which is lighter and easier for the pump to do (physics: path of least resistance). If that's the case, the you might have to think up of a solution/modification to the pump such that it has a plate over it (like a BD) and only sucks water lower to the bottom in a bigger radius.

I hope all this makes sense to you. Keep me updated.

P.s. **I hear that a lot of people around Malaysia and Indonesia use coral as a pH buffer. I'm not to sure about this b/c I heard that coarl is hard to clean, and they can contribute to some particular disease. Search around on this forum.