I do not know about the 250...but I have the 400 model and I think it must work the same...
Have been running in a 55
gal. tank for a month now, going thru first cycle.
You "MUST" have "bubbles" for the skimmer to work. If assembled correctly there should be a little tube coming out near top of filter with a small white cap inserted. This is the air intake valve and must be pulled out slightly to allow air into skimmer...making "bubbles" - more pulled out...more air...smaller the bubbles the better...
HOWEVER...IT IS VERY NOISY!...a well published "fix" is the placement of a 1" airstone at the base of the skimmer tower connected to a small "quiet" airpump...makes all the difference in the world and gets much nicer bubble action...
DONT BELIEVE ALL YOU HEAR ABOUT "SKILTER's"...they are underrated by most who have had little practical experience with them...they are the perfect starter filter system for those with little experience and without allot of $ to race out and buy a "Mercedes" system. Should make a great supplemental system if and when you decide to "grow" out of it...
RGDS
OmarD