My Red Sea 300 works great for my 125 / 55sump. With all rock and equipment, total volume is prob. 140-150. I've got a moderate bio load of 9 fish + cuc, feed generously at least once a day. Skimmer in combination with kessil H380 on fuge show almost undetectable readings in nitrate. 0 for ammonia and nitrite. All of this, including using 200 micron filter socks on the return into the sump and a UV sterilizer, the RS300 (once broken in, and it does take some time and tuning) still creates a thick, black skim in the skimmer cup which then (my favorite feature) can be drained via included drain tube and ball valve. Skimmer also includes an integrated neck squeegee to keep the skimmer neck clean and visible. The pump is not DC, but still very quiet. Other than sound, don't see the need for a variable speed DC pump. I want my skimmer running 100% all the time, unless im in feed mode, which it then shuts down for +5 minutes. Integral gate valve works well for tuning and adjustment is above water level so never need to get wet while breaking in or tuning!