Eclipse Hex 5 tank - what level should the water be filled up to?

The directions are unclear. At first, I think I did not put enough water because the biowheel was not moving. And then, I filled it up so the bottom of the biowheel casing is slightly submerged (about 1-2mm) and the wheel did move. I am worried that I should not submerge it though. Also - even when the biowheel moves, there seems to be very little surface movement...is this safe to house a goldfish? I used to us an airstone filter and there would be many bubbles at the surface...does the hex 5 aerate the water if I don't see bubbles? Thanks!