um.. i dont understand this q? to well.. u are asking how deep (summerged in water?) the tf1000 should be?? .. well if u look at the cr*p almost useless diagram... it's a -- skimmer and need a outside source to add new dirty water into it... so it dos'nt matter the deept the skimmer it in for it douse not use the sump/sump water lvl to proform... however u should add a valve to the line that provide the skimmer it's water..
(in the diagram the skimmer gets it water from a branch-off from the overflow.. i would put the valve in that line somewhere befor the skimmer.. just in case it's too much water for the skimmer to work correctly) however the way i set my tf1000 was i use the return pump.. i "t" off the return line... the 'T' branch off, valve, then UV, then my tf1000 return..and back into my sump.... my set-up is a lil weird but hey it workxxx