The problem is the area for the water to go back to the top is too small for such filter system.. later i try and see if can find pic for you to see. In the mean time, i think i will try and suck out as much debris from the bottom since it's just big pebbles so should be possible. Well, not much we can do as it's not design and built by us.. Otherwise, probably will be big enough and better design.
Yes, I saw the ranchus are mainly about S$500 onwards. But top view ranchu is very very nice and perfect for my setup. I probably won't keep ranchu for now.
Ya, know abt the overfeeding risk so trying hard to not overfeed. As for KOI, initially planned to but i understand KOI can be very long lifespan (longer than us!) and very huge so not suitable. Hence, the decision to keep giant oranda. I won't keep koi, maybe add in another giant oranda or 2. Thanks for the helpful advice.