A 400 pumps 400 gph. For a 120g you want more water turn-over than that (At least 6 times an hour) Of course, I am known for over-compensating when it comes to filtration. I would either go with two Emperor 400's and a large skimmer (Berlin's are generally very good) or consider the long term advantages of a wet dry (sump) filter. If you choose to go with the Emperors, make sure that you have a considerable amount of circulation (via powerheads) in the water so all the debris is directed towards a filter and there are no dead areas. Also, are you planning on having live rock or live sand? If so, you may be able to get by with hang on filters and a skimmer. Do you know what type of fish you are planning on keeping? This info might help me help you more. -David