DONT put the bm250 on a 155g!!!!!
that would be waaay too much overkill,...and that's coming from me, the skimmer freak.
think about it, we have a bm250 at barrier reef, it's on a 300g main display, with 180g frag tank, and 75g sump for a total amount of about 550g,
the tank stays pretty damn clean to be sure.
so i can say without a doubt, the ati bm200 can filter a normal fish load tank with mixed coral of up to 250g or an sps/heavily stocked 200g.
the only thing is, that the ATI bm200 is NOT for noobs, or lazy people!! you will have to adjust the airflow to accomodate your individual tank's skimming requirements, as well as have a chamber in your sump with a baffle where the water level never changes, and have to learn the tricks of this pump to deal with start up issues, as well as trimming/maintaining the mesh wheel impellar. BUT, it is a bad ass performer that produces a ton of skimmate, is dead quiet, it's cheap (if you compare it to other skimmers of the same power), and the short skimmer body allows it to fit in most aquarium stands. to me, unless your going to buy a royal exclusive bubble king, or ATB, the ATI skimmers are the way to go. but if your the kind of person who likes to set things and forget about them, then this skimmer is not for you. the other thing about the ATI's is that you get 1,100 lph of air for each 23 watt pump. try to find another skimmer that can beat that...
you might also want to look at the new octopus knock off's of the ATI:
http://www.marinesolutionsinc.com/catalog/index.php?cPath=22_48
i actually like that bubble plate design better, it's actually more like the bubble king than the bubble master skimmers. same sicce psk 2500 pump as the ATI's too. and that price...wow