My crushed coral is #3 (not so fine). I can leave gaps for water and dirt to pass through under the undergravel filter. The pump suck the water from below the filter so I guess everything will be sucked up?

First I adjuested the water level to the water line mark on the skimmer cylinder but I read somewhere it should be right to the rim of the collection cup. I did that 10 mins ago.

I am using Ataman 2000l/hr pump for my weipro 2012. Understand it need 400 gal/hr pump rate. I did a conversion, seem to be around there but not much foam leh.