Depends on what other filtration you have. Lets say you have a double cartridge whisper or penquin filter on the back, live sand and live rock.
Example:
Lets say you put a Penguin 330 rated for 30-60 gallon tanks.
This puts out 330gph add two MJ 600's @ 160 gph/per
This would give you 650 gph divided by your 30 gallon tank = 21.6666 gph, a bit to fast IMO, others might not think so. Remember if it is a reef tank then you would not actually have 30 gallons of water either. So it all depends on what other (if any) forms of mechanical filters you use. I assume that most of us want to run carbon once in a while, thus my example has a saddle or power filter in the equation.
Let us know what you were thinking about in regards to filtration.
Thomas